Integrating living mulches into irrigated cropping systems may benefit producers in the western U.S. Mulch crops can be successfully co-established with corn or oats. White clover shows potential as a living mulch due to its positive effects on corn grain and silage yields when adequately suppressed. It also has high glyphosate tolerance. The combination of strip-tilling and use of glyphosate herbicide appears to be the most effective strategy for suppressing living mulches. Leguminous living mulches can reduce nitrogen fertilizer needs, but adequately suppressing the mulch to minimize annual crop yield losses while maintaining the perennial legume stand remains a challenge.
1. Determine methods of establishing various perennial plant species potentially adapted for use as living mulches under irrigation.
2. Evaluate methods of suppressing living mulches to avoid reduced yields of associated crops.
3. Quantify the environmental and economic benefits of using living mulch systems under irrigation.
4. Demonstrate the benefits of using living mulch systems for crop production under irrigation to producers through on-farm trials.
Increasing input costs along with environmental conservation issues have created the need for agricultural research in the area of low-input, sustainable cropping systems. Cover cropping, tillage reduction and value-added crops have drawn a great deal of focus in addressing both environmental and economic concerns. One concept that embodies and expands upon such ideas is that of a living mulch.
Living mulches are cover crops grown in association with an annual cash crop (Paine and Harrison, 1993; SAN, 1998). These vegetative covers are unique in that they are not completely killed prior to planting of the annual crop like a traditional green manure or cover crop. Rather, growth is temporarily suppressed allowing eventual persistence and coexistence of the cover with the annual crop throughout the growing season and beyond (Echtenkamp and Moomaw, 1989; Singer and Pederson, 2005). These mulches can be annuals or perennials and can be interseeded with the cash crop or established before planting (Singer and Pederson, 2005). The use of a perennial cover offers many potential benefits, including decreased wind and water erosion, increased water infiltration, weed suppression, reduced insect damage and increased soil organic matter (Echtenkamp and Moomaw, 1989; White and Scott, 1991; Hartwig, 2004). Another advantage of living mulches is improved nutrient cycling. All covers provide some nitrogen retention by limiting nitrate leaching (Duiker and Hartwig 2004). However, leguminous living mulches offer the greatest fertility improvements through the biological fixation of atmospheric nitrogen.
Aside from the primary benefits discussed above, legumes are also highly palatable and increase the forage quality of grazed crop aftermath, such as corn stover, by supplementing protein and energy (Zemenchik et al., 2000). This gives livestock producers the ability to substantially increase the feed value of forage they may already be grazing. In some cases, there is also potential for spring grazing or harvest before the cash crop is planted.
Several recent field studies on the use of leguminous living mulches for corn production in the upper Midwest of the U.S. have yielded positive results showing significant nitrogen additions and subsequent corn yield responses. Albrecht et al. (2009) found that N additions beyond 22 kg ha-1 did not induce a yield response in corn grown with a kura clover (Trifolium ambiguum) living mulch. This would suggest that the majority of the N requirement was met by the clover. Studies by both Zemenchik et al. (2000) and Affeldt et al. (2004) indicated that corn planted into an established kura clover living mulch required little to no addition of N fertilizer and experienced no yield reduction.
Conversely, Sawyer et al. (2010) did not report any significant reduction in the N requirement of corn planted into kura clover in northeast Iowa. A lack of yield response to N fertilization was only found when corn growth was already unacceptably limited by competition with the living mulch. Duiker and Hartwig (2004) also found that N fertilizer could not be reduced in the presence of leguminous living mulches without decreasing corn yield. In this case, N contribution by the legumes was only observed at severely deficient levels when corn yield was already depressed by N deficiency. Their trials in southeastern Pennsylvania included crown vetch (Coronilla varia), flat pea (Lathyrus sylvestris), birdsfoot trefoil (Lotus corniculatus), hairy vetch (Vicia villosa) and galega (Galaega officinalis).
Though less research has been conducted on the potential of living mulches for soybean production, initial results indicate that seed yields are reduced in a living mulch system. Pederson et al. (2009) found that yields in living mulch systems under three different suppression regimes were lower than those in a clean-till system in which the kura clover cover crop was completely killed prior to planting of the annual.
These variable results highlight the need for additional research to determine optimal management practices for living mulches. Practices will vary based on the production goals of individual growers and the environments in which they are operating.
While living mulches have been tested extensively under rain-fed conditions (Eberlein et al., 1992, Affeldt et al., 2004; Duiker and Hartwig, 2004), there is a lack of published data on their use in semi-arid environments that require irrigation. Most of the research to date has come from the upper Midwest and eastern United States. Soil types, climatic conditions, insects, weeds and disease pressure in the semi-arid West differ from these humid regions. Thus, research on species selection and general management practices for living mulches must be conducted in the region if the system is to be adopted in the West.
One major obstacle to the adoption of living mulches by grain producers in the West is the economic cost of establishing the cover. Since land is a major input cost for most farmers, losing a year of production is often not a viable option. A potential solution to this problem is co-establishing the living mulch with an annual cash crop. The two crops can be planted at the same time, with the living mulch being allowed to persist for future use after harvest of the annual (Paine and Harrison, 1993). Of course, there are obvious drawbacks to this strategy. Competition with the annual crop will hinder establishment of the cover, particularly in the case of crops that form a thick canopy depriving the smaller cover crop of light. Another issue is the reduction in weed control options. Herbicide treatments may be severely limited when intercropping a grass with a legume. While established perennial legumes can be resilient to herbicide applications, they will be susceptible and easily killed during the establishment year. Thus, careful selection of both the cover crop species and the companion annual with which it is seeded are essential to the success of establishment.
This project was conducted at multiple locations within Colorado that represent diverse environments in both the eastern and western parts of the state. Results from the eastern Colorado locations are applicable to the High Plains region (especially eastern Colorado, eastern Wyoming, western Kansas and western Nebraska), while results from the western Colorado locations apply to many areas of the Intermountain region.
Objective 1 – Determine methods of establishing various perennial plant species potentially adapted for use as living mulches under irrigation.
Two fields were used for co-establishment studies in 2009. The study sites were located at the Colorado State University Agricultural Research, Development, and Education Center (ARDEC) about 6 km south of Wellington, CO at an elevation of 1554 m. Both were irrigated, one with a linear drive sprinkler system and the other by furrow irrigation from gated pipe. All subsequent references to the two sites will be made by the type of irrigation they received.
The sprinkler irrigated field was initially split into three sections to be used for three different annual crop treatments. These included corn, spring oats and corn interseeded with spring oats. Each section was arranged in a randomized block design with four replications and included three legume treatments (birdsfoot trefoil, white clover, and mix of kura, red and white clovers) and a control with no living mulch. Plot dimensions were 4.6 x 10.7 m. ‘Morton’ spring oats were seeded on May 8, 2009 at 22.4 kg/ha using a no-till drill. Row spacing was set at 19 cm, and planting depth was 2.5 cm. Legumes were seeded the following day using the same drill. Seeding depth was adjusted to approximately 1 cm. Grand Valley Hybrids ‘22R77P’ Roundup Ready hybrid silage corn was planted on May 15, 2009 using a 6-row John Deere corn planter. Row spacing was 76 cm, and planting depth was 3.8 cm. The planting population was 76,600 seeds/ha. Oats were seeded east to west, while legumes and corn were seeded north to south. The study site was irrigated with a linear drive sprinkler system. Irrigation began on May 20, 2009 and continued throughout the growing season on a weekly basis. The quantity of water was adjusted to meet crop needs.
A hail storm on June 10, 2009 caused significant damage to the corn. Oats and legumes were also damaged to a lesser extent. The corn in the corn-only strip was able to recover, but the corn intercropped with oats was unable to compete with the oats and remained stunted. On June 29, 2009, the corn strip was side-dressed with 168 kg/ha nitrogen using 32-0-0. At that point, it was deemed unnecessary to side-dress the corn in the corn/oat strip due to its extremely poor performance. Instead, the strip was harvested for oat hay.
The corn/oat strip was harvested for hay at the boot stage of the oats on July 8, 2009. A self-propelled swather set at a cutting height of 10 cm was used for harvest. A 2.8 m wide by 6.1 m length of windrow was collected onto a large tarp and weighed with a hanging scale to determine bulk yield. A subsample was taken to determine percent moisture and adjust to dry matter (DM) yield. The oat strip was harvested at the soft dough stage on July 27, 2009. The aforementioned harvest techniques were used.
The corn strip was harvested on September 29, 2009 using a standard pull type two-row silage chopper. Chopped silage was blown into a silage truck with a weigh body. Weights were recorded for the middle two rows of each six-row corn plot. These weights were used to determine bulk yield. Two subsamples were taken with a net by periodically putting it under the chute of the silage chopper as the plot was cut, one to ensile while the other one was dried and used to determine percent moisture in order to calculate yield on a DM basis.
The furrow irrigated field was leveled and set with irrigation furrows on 76 cm centers the previous year. Legume treatments and plot dimensions were identical to those described at the sprinkler irrigated site. Initially, three annual crop treatments: oats, corn and a control with no annual crop were planted. However, herbicide carryover effects and non-uniform soil fertility led to extremely uneven legume establishment in the corn and legume-only blocks. Consequently, only the oat treatment could be used. The oat test was set up in a randomized block with four replications. Each block contained three plots of each legume treatment and only one control plot (no legume). This design was used in anticipation of a study to be conducted at the site the following year.
The legumes and oats were seeded on May 22, 2009 using a modified cone-seeder drill. The planter was adjusted such that each bed had three rows of oats spaced at 16.5 cm between rows. ‘Morton’ spring oats were planted first at a rate of 22.4 kg/ha. Planting depth was 2.5 cm. Legumes were seeded using the same row spacing, but planting depth was adjusted to approximately 1 cm. The result was that oats and legumes were planted in the same row. The site was furrow irrigated on an as needed basis.
Oats were harvested for hay on July 24, 2009 at the soft dough stage. This was done using a flail chopper with an attached weigh bin to determine bulk yield. Cutting height was set to 10 cm, and harvest data were obtained from the middle two beds of each 6-row plot. A subsample was taken from each plot by using a net to catch plant material as it was blown into the weigh bin. One was dried to determine percent moisture, which was used to calculate yield on a DM basis. The other was separated to determine percent weed, legume and oat composition.
Subsamples taken to determine percent moisture were weighed immediately after harvest in the field. They were placed in a forced air oven set to 55oC for a minimum of 72 hours. After drying, samples were weighed again to calculate percent moisture, which was used to determine DM yield.
Corn samples were ensiled by putting 600 g into a 3.8 L plastic bag. Air was evacuated from bags, and bags were then heat sealed using a kitchen-grade vacuum sealer kit. Sample bags were stored in black plastic trash bags at room temperature for 90 days before being opened and dried.
Dried samples from all three annual crop treatments (ensiled in the case of the corn) were ground using a Wiley mill in preparation of running forage quality analyses. Neutral detergent fiber (NDF), acid detergent fiber (ADF) and total nitrogen content were determined for all oat hay and corn silage samples taken at time of harvest. DF/ADF fiber analyses were performed using the ANKOM filter bag technique. This method involved putting ground samples in filter bags which were sealed and digested in an ANKOM fiber analyzer. Nitrogen concentration was determined using a LECO carbon and nitrogen analyzer. The percent nitrogen concentration was multiplied by 6.25 to estimate crude protein (CP).
For the sprinkler irrigated site, PROC MIXED in SAS was used to determine the effect of legume treatment on annual crop yield, NDF, ADF and CP. Annual crop yield and quality factors were compared within each annual crop but not between crops. These data were analyzed as a randomized block design. Differences were recognized as significant at the P < 0.05 level. If legume treatment effect was found to be significant, treatment means were separated using LSMEANS.
For the furrow irrigated site, oat hay yield was analyzed as a randomized block with subplots. Each legume treatment had three subplots, and the conventional (control) treatment had only one. For NDF, ADF and CP of oat hay, one plot of each treatment was randomly selected from each replication. These data were analyzed as a randomized block. All data were analyzed using PROC MIXED. If legume treatment effect was significant at the P < 0.05 level, treatment means were separated using LSMEANS.
Objective 2 – Evaluate methods of suppressing living mulches that both conventional and organic producers can use to avoid reduced yields of associated crops.
Study 1 –
Field testing of established living mulches was conducted at the Colorado State University Western Colorado Research Center (WCRC) at Fruita, Colorado. The WCRC is at an elevation of 1,375 m and gets approximately 21 cm of precipitation annually. The field was irrigated with furrows set on 30 in centers. Water was applied throughout the growing season to meet annual crop needs.
The study site had small plots of well-established legumes that were planted in the spring of 2007. The field was composed of three annual crop strips, each of which was arranged in a randomized complete block with four replications. Strips were seeded with corn for grain, corn for silage and soybeans in the spring of 2009. Treatments within each strip included five different living mulches composed of varying legume species/combinations and four plots without legumes that were used for fertility treatments. Legume treatments were ‘Focus’ alfalfa (Medicago sativa), ‘Starfire’ red clover (Trifolium pratense), ‘Kopu II’ white clover (Trifolium repens), ‘Norcen’ birdsfoot trefoil (Lotus corniculatus), and a mix of red clover and birdsfoot trefoil. Fertility treatments included 0, 84, 168 and 252 kg N/ha, which was applied midseason. Variable N rates were not applied to the soybeans, since they do not commonly receive midseason nitrogen fertilization. Instead, this strip had four control plots with no mulch in each replication. Plot dimensions were 4.6 x 15.2 m.
All plots were evaluated on February 27, 2009 to ensure that legume stands were adequate and uniform across species. Plots were rated on a scale of 1 to 5. Legume aerial phytomass samples were obtained from the soybean strip on April 27, 2009 by randomly placing a circular hoop with a diameter of 67.3 cm into the center of each plot and clipping all plant material in that area to ground level. These samples were dried, weighed and used to calculate above ground biomass on a per acre basis.
Pre-plant fertilization consisted of a broadcast application of mono-ammonium phosphate (MAP, 11-52-0) to achieve 116.5 kg P2O5/ha. This was applied on April 14, 2009. The fertility plots, those without a living mulch, were sprayed with glyphosate at a rate of 3.8 kg a.e./ha on April 19, 2009 to ensure that they were free of any legumes. This was done using a CO2 pressurized backpack sprayer. The following day, glyphosate was broadcast applied across all plots at a rate of 1.3 kg a.e./ha using a tractor-mounted boom sprayer. This reduced rate of glyphosate was applied as a means of living mulch suppression prior to planting of annual crops. The furrows were cleaned the next day to ensure adequate flow of irrigation water. On May 4, 2009, the field was strip-tilled using an Orthman 1tRIPr two-row model. Tilled swaths were 25 cm wide on 76 cm centers, leaving approximately 50 cm untilled between strips. Upon visual evaluation, it was decided that tillage was inadequate for planting of annual crops, particularly in the white clover plots which had formed a thick, stoloniferous mat. As a result, all plots were strip-tilled a second time the following day, with white clover plots being tilled a third time.
Surprisingly, the white and red clover appeared relatively unaffected by the earlier glyphosate application, while the alfalfa and birdsfoot trefoil had no visible green tissue. For this reason, a broadcast application of paraquat at a rate of 0.8 kg a.i./ha was made on May 5, 2009. This was intended to ensure adequate suppression of mulches and allow time for early growth of annual crops.
Corn for grain (Grand Valley Hybrids GVH 22R77P) and corn for silage (Grand Valley Hybrids GVH 23T53P) were planted on May 7, 2009 at 88,200 seeds/ha. Soybeans (Northrup King S-28-B4 02RM018047) were planted on May 29, 2009 at 379,000 seeds/ha. Corn and soybeans were planted at 5 and 4 cm depths, respectively, using a Buffalo no-till planter.
Nitrogen fertility treatments were side-dressed when the corn was at the V6 to V7 growth stage. This was done by dribbling a urea ammonium nitrate (UAN, 32-0-0) solution on either side of each corn row through a rolling fluted coulter equipped with a fertilizer drop tube. All living mulch plots were fertilized at a rate of 84 kg N/ha, while fertility plots received their designated rates.
A midseason herbicide application was deemed necessary in the soybean plots due to competition from the living mulches. Glyphosate (0.6 kg a.e./ha) was broadcast applied with a boom sprayer on July 14, 2009. No midseason herbicide applications were made on the corn grain or silage plots.
Corn silage was harvested on September 9, 2009 with a standard pull-type, two-row silage chopper. Chopped silage was blown into a silage truck with a weigh body. Weights were recorded for the middle two rows of each six-row corn plot. These weights were used to determine bulk yield. Samples were taken with a net by periodically putting it under the chute of the silage chopper as the plot was cut. A subsample was ensiled and later used to determine percent moisture to calculate yield on a dry matter (DM) basis.
Soybean plots were harvested on October 17, 2009 using a Hege plot combine. Weights were taken from the center two rows of each plot, and subsamples were used to determine moisture. All yields were adjusted to a seed moisture content of 12%.
Corn was harvested for grain on October 23, 2009 with a modified Gleaner plot combine. Plot yields for the center two rows were recorded and adjusted to a moisture content of 15.5%.
Corn residue and legume biomass were collected from corn grain plots on November 10, 2009. This was done by laying a 76 x 76 cm frame flat on the ground at random locations in the center each plot. The frame was centered on a bed so that it covered the area from one furrow to the next. All corn residue and legume biomass within this area was collected by cutting at the ground level. Material was cut at the edge of the frame using hand clippers to ensure that only residue within the frame was taken. Corn residue was later separated into leaf, stem and cob components, dried and weighed. Legume residue was also dried and weighed.
Spring legume biomass samples were dried to a constant weight in a forced air oven at 55oC for a minimum of 72 hours. Dry weights were used to calculate potential spring legume yield on a DM basis.
Corn silage samples were ensiled by putting 600 g of wet material into a 3.8 L plastic bag. Air was evacuated from bags, and bags were heat sealed using a kitchen-grade vacuum sealer kit. Sample bags were stored in black plastic trash bags at room temperature for 90 days before being opened and dried. After removal from the sealed bags, silage samples were immediately dried in a forced air oven at 55oC for a minimum of 72 hours. Samples were weighed to calculate percent moisture, which was used to determine DM yield.
Dried legume and corn silage samples were ground through a Wiley mill. Neutral detergent fiber (NDF), acid detergent fiber (ADF) and total nitrogen content were determined for all legume and corn silage samples. NDF/ADF fiber analyses were performed using the ANKOM filter bag technique. This method involved putting ground samples in filter bags which were sealed and digested in an ANKOM fiber analyzer. Nitrogen concentration was determined using a LECO carbon and nitrogen analyzer. The percent nitrogen concentration was multiplied by 6.25 to estimate crude protein (CP).
Moistures for corn grain and soybean samples were determined using a DICKEY-john seed analyzer. These were used to calculate yields by adjusting to the appropriate moisture content.
Due to an observed lack of recovery by many of the legumes, all plots were evaluated on April 27, 2010 to determine stand loss. Plots were rated on a scale of 0 to 5 with 0 indicating no remaining legumes and 5 being a full, healthy stand.
Statistical analyses were performed using PROC MIXED in SAS. Legume establishment and mortality ratings were compared within and among annual crops. These data were analyzed as a split-plot design where the whole plot was the annual crop, and the split plot was the legume treatment. PROC MIXED was used to determine main effects and interactions of legume treatment and annual crop. All yield, quality and residue data were analyzed as a randomized block design with replication as the random effect and legume/fertility treatment as the fixed effect. These comparisons were made within a given annual crop, not among crops. Differences were recognized as significant at the P > 0.05 level. When aforementioned effects were found to be significant, treatment means were separated using LSMEANS.
Nitrogen fertilizer equivalencies for legume treatments were determined by means of an inverse prediction based on crop yield response to nitrogen fertility treatments. This response was based on a linear regression generated in Microsoft Excel. Accordingly, confidence intervals for these estimates were calculated.
Study 2 –
Two fields were used to test different chemical suppression regimes in the spring of 2010. Legumes at these sites were seeded in the spring of 2009 as part of the co-establishment studies outlined under objective 1. Legumes tested at both sites included birdsfoot trefoil, white clover, and a mix of red clover, kura clover and white clover. The sprinkler irrigated site consisted of 4.6 x 10.7 m plots, which were split into three 16-plot blocks. These were used as the three replications for this study. Blocks had been seeded with corn, spring oats and corn interseeded with spring oats the previous year, during which all legumes successfully established. The field was laid out in a split plot design where the spray treatment was the whole plot and the legume treatment was the split plot. Variable rates of glyphosate (1.0, 1.5, and 2.0 kg a.e./ha) and paraquat (0.7 kg a.i./ha) were applied in the spring in order to suppress the mulches prior to corn planting.
Legume treatments and plot dimensions at the furrow irrigated site were identical to those described at the sprinkler irrigated site. However, the experimental design differed. In this case, the field was laid out in a randomized block design with four replications. The glyphosate rates were the same as the sprinkler irrigated site, but there was no paraquat treatment. This site also differed in that it only had one treatment with no legumes, which was considered the control. This was treated with glyphosate at 2.0 kg a.e./ha for weed control. Herbicides were applied using a CO2-pressurized backpack sprayer with wide angle flat spray tips. All glyphosate rates and paraquat (sprinkler irrigated site only) were applied with 94 L water/ha. Glyphosate concentrations were adjusted in mix bottles, and ground speed was kept constant. The center 3.0 m of each plot was sprayed. After applying treatments to the middle of each plot, the outer edges were sprayed with glyphosate at a rate of 1.5 kg a.e./ha. Birdsfoot trefoil, white clover and the clover mix were approximately 10, 12 and 25 cm tall, respectively.
Both sites were strip-tilled on May 24, 2010. An Orthman 1tRIPr six-row model was used to till 33 cm strips on 76 cm centers at the sprinkler irrigated site. An Orthman 1tRIPr two-row model was used at the furrow irrigated site to till 25 cm strips on 76 cm centers. A narrower tilled strip was required at the furrow-irrigated site in order to maintain some legume growth on the edges of the raised beds. Producers Hybrids ‘5004VT3’ glyphosate resistant corn and Croplan Genetics ‘421RR2’ glyphosate resistant corn were planted at the sprinkler and furrow irrigated sites, respectively, immediately after tilling. Corn was planted to a depth of 3.8 cm at a population of 80,300 seeds/ha using a 6-row no-till corn planter. Planting into the tilled strips ensured that six rows of corn were planted in the width of each plot, with the center four of those falling in the treated area.
Irrigation at the sprinkler irrigated site began on June 2, 2010 and continued throughout the growing season on a weekly basis. The quantity of water was adjusted to meet crop needs. The furrow site was irrigated on an as needed basis beginning on June 4, 2010.
Corn was side-dressed with 112 kg N/ha in the form of 32-0-0 on June 21, 2010. A mid-season herbicide application was also deemed necessary due to weed pressure. This was made on July 6, 2010 as a broadcast application over all plots. Glyphosate was applied at a rate of 1.0 kg a.e./ha with 150 L water/ha using a tractor-mounted boom sprayer.
Due to severe bird damage that occurred in some areas of both study sites late in the season, all plots were rated for percent loss to bird damage. This was done by randomly selecting ten ears from the middle two rows of each plot and estimating the percent of total grain lost from each ear. All the percentages for a given plot were then averaged and used to adjust plot yields to estimate full yield potential in the absence of bird damage. While field averages for bird damage were relatively low (16 and 8% at the sprinkler and furrow irrigated sites, respectively), this was deemed necessary due to variability in the extent of damage across the plot areas. Bird damage ratings were recorded immediately prior to corn harvest.
Corn was harvested for grain on October 27, 2010 using a combine equipped with a yield monitor. Three of the center four rows were harvested from each plot. Plot yields were recorded and adjusted to a moisture content of 15.5%.
Residue was collected from corn grain plots on October 28-30, 2010 and November 4-6, 2010 at the furrow and the sprinkler irrigated sites, respectively. This was done by laying a 76 x 76 cm frame flat on the ground in the center of each plot. The frame was centered on a bed so that it covered the area from one furrow to the next. All corn residue, legume and green weed biomass within this area was collected. Desiccated or partially decomposed legume and weed material was discarded. Material was cut at the edge of the frame using hand clippers to ensure that only residue within the frame was collected. Legumes and corn stubble were cut at ground level using hand clippers. Corn residue was later separated into leaf, stem and cob components. Corn components, legumes and weeds were dried to a constant weight in a forced air oven at 55oC for a minimum of 72 hours. Dry weights were then used to calculate corn residue, legume biomass and weed biomass on a per hectare basis.
At the sprinkler irrigated site, PROC MIXED in SAS was used to determine the effect of legume and spray treatments on annual crop yield, corn residue components and fall legume and weed biomass. Data were analyzed as a randomized block design with a split plot treatment structure where spray treatment was the whole plot, and legume treatment was the split plot. Block and block interactions were random effects. Legume and suppression treatments were considered fixed effects. Treatments with no legume were omitted from comparisons of legume residue. Differences were recognized as significant at the P < 0.05 level. If legume or suppression treatment effects were found to be significant, treatment means were separated using LSMEANS.
At the furrow irrigated site, PROC MIXED in SAS was used to determine the effects of legume and spray treatments on annual crop yield, corn residue components and fall legume and weed biomass. Data were analyzed as a randomized block design. The control treatment (with no legume) was omitted. Block was the random effect, and legume and suppression treatments were considered fixed effects. Dunnett’s Test was then used to compare each individual treatment (legume/suppression combination) to the control. Differences were recognized as significant at the P < 0.05 level. If legume or suppression treatment effects were found to be significant, treatment means were separated using LSMEANS.
Study 3 –
Glyphosate rate studies were conducted in the field and using potted plants started in the greenhouse. Both studies tested the same five legume species: alfalfa, birdsfoot trefoil, kura clover, red clover and white clover. All legumes were treated with the proper Rhizobium inoculants prior to planting. The field study subjected each legume to four rates of glyphosate, while the pot study included three rates.
The field study site was located at Colorado State University’s Agricultural Research, Development, and Education Center (ARDEC). ARDEC is located about 6 km south of Wellington, CO at an elevation of 1554 m. While ARDEC receives 33 cm of precipitation annually, the site was irrigated with a linear drive sprinkler system on an as-needed basis in both the fall of 2009 and spring of 2010.
The field was sprayed with glyphosate at 1.3 kg a.e./ha and mowed on August 2, 2009. It was then sprayed with glyphosate at the same rate a second time one day prior to planting to ensure a weed-free seedbed. Legumes were seeded on August 18, 2009 using a no-till drill. Row spacing was set at 19 cm and planting depth was approximately 1 cm. Legumes were irrigated weekly through September 16, 2009.
The site was laid out in a strip plot design with four replications. Legume species were randomized within each replication, and spray treatments were applied perpendicular to the direction of legume strips. Each plot was 2.1 x 1.5 m. The herbicide treatments were applied on May 18, 2010 using a CO2-pressurized backpack sprayer. The boom was fitted with wide angle flat spray tips. Spray treatments included glyphosate applied at 1.0, 1.5, 2.0 and 2.5 kg a.e./ha. All rates were applied with 94 L water/ha. Glyphosate concentrations were adjusted in mix bottles, and ground speed was kept constant. Plots were irrigated on a weekly basis starting on June 2, 2010 and continuing for the duration of the study.
Above ground biomass samples were taken eight weeks after herbicide application. Plot yields were determined by taking a 0.25 m2 frame from the center of each plot. All plants were clipped to the ground level, weeds were removed and samples were dried to a constant weight for 72 hours in a forced air oven at 55oC. Dry weights were then recorded. Plot yields were taken again 16 weeks after glyphosate applications using the aforementioned technique. In this case, weeds were separated and dried as well.
The pot study was started in the Colorado State University Greenhouse Facility. Legumes were seeded on November 7, 2009 into 9 x 9 x 13 cm plastic pots filled with potting soil. They were thinned to five plants per pot on November 18, 2009. All plants were checked daily and watered on an as needed basis. On February 4, 2010, legumes were transplanted into larger round pots (16 cm diameter by 16.5 cm deep) filled with field soil. This soil was collected from ARDEC in a field adjacent to the one in which the field study was conducted. Excess potting soil was removed by shaking plants and gently pulling roots apart. Plants were transferred to field soil due to documented differences in efficacy of glyphosate in sterile potting soil vs. unsterile field soil. It also offered a greater degree of continuity between the field and pot studies. All legumes were fertilized with 1 g of triple superphosphate per pot on February 15, 2010.
To simulate winter dormancy that these perennial species would undergo in the field, plants were gradually hardened off and moved outside. On February 25, all legumes were clipped to a height of 10 cm and moved from the main greenhouse bay to a ventilation corridor where temperatures varied from 5 to 19oC depending on when ventilation fans were running. One week later, they were moved to a walk-in cooler and maintained at a temperature slightly above freezing (1 to 3oC) with no light. On March 11, plants were moved to a fenced off area adjacent to the university greenhouses where they were exposed to outdoor temperatures and weather conditions. In this area, they were under 50% shade and were allowed to break dormancy under natural seasonal climate change.
Herbicide treatments were applied using a moving nozzle spray chamber equipped with a single nozzle. Glyphosate was applied at rates of 1.0, 1.5 and 2.0 kg a.e./ha. There were three replications of each treatment and three untreated control pots for a total of 12 pots of each species per set. There were two sets of plants with the first set being sprayed on April 20th and the second on April 22nd.
Approximately eight weeks after herbicide application, all plants were clipped to ground level, dried to a constant weight for 72 hours in a forced air oven at 55oC and weighed. Remaining plant roots (and crowns) were washed over a 6.35 mm screen, dried and weighed. All rhizomes and stolons were included with root/crown material.
For the field study, above ground legume biomass dry weights for the different glyphosate treatments were analyzed as a percent of the untreated control, allowing for a comparison between the two sampling dates. Data were analyzed as a strip plot design. PROC MIXED in SAS was first used to determine main effects and interactions of legume species, spray rate and sampling date. Replication, replication x date, replication x legume and replication x spray rate were considered to be random. In the event of a three-way interaction, data were separated by sampling date and analyzed to determine main effects and interactions of legume species and spray rate. Differences were recognized as significant at the P < 0.05 level. When aforementioned effects were found to be significant, treatment means were separated using LSMEANS.
Kura clover had to be analyzed separately due to an irregularity in the control treatment. The kura seed was contaminated with red clover seed. The red clover, which has greater seedling vigor, outcompeted the kura clover in the control treatment causing very low kura clover yields. However, the kura proved to be much more glyphosate tolerant and outcompeted the red clover in all of the spray treatments, resulting in little to no red clover contamination. Thus, the four kura treatments were compared on an actual biomass basis rather than as a percent of the untreated control and could not be compared with the other species. Within each species for a given sampling date, legume biomass was regressed linearly against spray rate using Microsoft Excel. PROC MIXED was used to determine whether the correlation between legume and spray rate was significant (p?0.05).
For the pot study, final above ground and root/crown biomass for all spray treatments were converted to a percent of the untreated control and analyzed using PROC MIXED to determine main effects of legume species and spray rate. Differences were recognized as significant at the P < 0.05 level. When aforementioned effects were found to be significant, treatment means were separated using LSMEANS. Regression analyses were performed on root and shoot biomass using the same methodology as in the field trial.
Objective 3 – Quantify the environmental and economic benefits of using living mulch systems under irrigation.
Due to the short-term nature of this project and the difficulty in adequately suppressing the living mulch without killing it, we were not able to gather enough data to address this objective.
Objective 4 – Demonstrate the benefits of using living mulch systems for crop production under irrigation to producers through on-farm trials.
A one-acre field of living mulch was established at Matsuda Farms which is located northwest of Wellington, CO. A mix of kura, red and white clover was seeded using a Brillion seeder in the spring of 2008 and allowed to establish for one growing season. This is an organically certified farm which negated the use of any herbicides for weed control. During the first growing season, the plot was mowed twice with a flail mower to control weeds.
In 2009, the initial spring growth of clovers was harvested for hay. The field was then strip tilled using an Orthman 1tRIPr. The strip tiller created a 25 cm wide seedbed down the middle of each irrigation bed. This particular field was furrow irrigated on 75 cm centers. Foxtail millet was then drilled into the tilled strips using a Great Plains drill on 19 cm centers. A hay crop was harvested later in the summer followed by the field being strip tilled again and planted to winter triticale. In the spring of 2010, the triticale-clover mix was harvested for hay. The field was immediately strip tilled and planted to sorghum-sudangrass which was harvested later in the summer for hay.
Because our second producer withdrew from the project, we established a demonstration seeding at the Western Colorado Research Center at Fruita. This was a co-establishment study in which we first seeded a mix of red, white and kura clover on the furrow irrigation bed using a Brillion seeder followed by planting of corn for grain. A strip of granular Harness herbicide was applied down the middle of the irrigation bed with the corn planter at time of seeding to control weeds and keep any clover from establishing in the center of the bed. A Buffalo brand no-till planter was used to plant the corn. This demonstration was repeated in 2010 except that the Buffalo planter was modified to broadcast a mix of red clover, white clover and birdsfoot trefoil seed on the irrigation bed, apply the granular herbicide and seed the corn all in one pass.
Objective 1 –
Legume treatment did not have an effect on the yield or quality of annual crops. This is not surprising as legumes were not well established at the time of harvest due to intense competition for light, nutrients and water from the faster-growing annual grasses. In the case of the oats, both early and late cuttings, the legumes did not contribute to crop yield. While there were legumes present in both treatments at harvest, they did not have enough growth to be harvested at the 10 cm cutting height. In the case of the corn silage, biomass samples taken prior to harvest indicated that birdsfoot trefoil and the clover mix would contribute to biomass harvested at a 10 cm cutting height. These legume treatments accounted for 0.8 and 0.6% of total silage dry matter, respectively. The two treatments were not significantly different but were both found to be greater than the white clover treatment. Still, such a small contribution would not be expected to affect the feed value of the silage.
It should be noted that the primary goal in co-establishment of a living mulch with an annual crop is not to reap the full, long-term benefits of the mulch. Rather, it is to successfully establish the perennial without negatively affecting the annual cash crop. In this case, none of the legume treatments reduced productivity of the oats or corn when compared to the conventional treatment with no living mulch. This indicates that the only costs associated with establishing the perennial cover are those of seed and planting. That being the case, there is very little risk to the producer involved in establishing a living mulch.
The one notable challenge found in this study was that of weed control. Intercropping a grass with a broadleaf, especially in the establishment year, makes weed control difficult. This did not prove to be a major issue in the oat plots, which quickly formed a dense canopy and outcompeted the weeds. However, in the case of the early cut oats, the clover mix did have more weeds as a percent of total harvested biomass, which is surprising as good legume establishment should suppress weeds. Conversely, the corn was plagued with high weed populations that contributed significantly to total biomass at harvest time. However, weed pressure did not differ between mulch treatments and the control.
This does raise an issue in the management of our control or “conventional” corn treatment in that a producer would likely spray when weed populations grew as high as they were in this study. We did not spray so as not to introduce another variable that would keep us from quantifying any weed suppression by the legume treatments. One recommendation from the 2009 results of this study is that a site with relatively low weed pressure should be chosen for establishment of the mulch. If such a field is not available, the use of an annual that provides rapid cover and vigorous growth is advisable.
There was an annual crop effect on the success of legume establishment. While there was no difference between the oat cuttings, they both outperformed the corn in the visual evaluations (Table 1.1) and the amount of legume biomass (Table 1.2). This is due to a much longer period of competition-free growth the previous year. However, there was one exception in the case of the visual ratings in which the clover mix ranked the same in the corn and late-cut oats. The clover mix established better than the white clover and birdsfoot trefoil under the corn. However, the trefoil established better than both the clovers under the late-cut oats. This can be attributed to damage that voles caused to the clover in the oat plots.
Vole damage was much greater in the oat plots compared to the corn silage. This damage occurred between fall 2009 and early spring 2010. Early- and late-cut oats were harvested on July 8 and July 27, 2009, respectively, allowing ample time for competition-free clover growth after harvest. Conversely, corn was harvested on September 29, 2009 leaving very little time for clover regrowth before winter. The resulting superior clover growth after harvest in the oat plots likely provided an excellent source of food and shelter for the voles.
Legumes are known to be a preferred food source for voles (Thompson, 1965). Meadow voles (Microtus pennsylvanicus) in an enclosed environment favored legumes from among 30 potential food sources. Of these, white clover and red clover (both of which are in the clover mix) were ranked first and third, respectively. Aside from a food source, the cover offered by live clover in the fall and spring, as well as a thick mat of residue in the winter, would create ideal rodent habitat. Previous research has shown that cover crops are excellent rodent habitats that can increase pest pressure (Sullivan et al., 2001). Winman et al. (2009) found that living mulches containing legumes attracted larger vole populations than those without. The clover mix had the most biomass in the fall and consequently left more residue behind.
Thus, vole damage is the most likely cause of this annual crop by legume treatment interaction. Additionally, clover mix establishment in the oat strips tended to be very strong in areas not affected by voles. It is possible that a late fall harvest or grazing of the clover could have made for less enticing rodent habitat.
Legume biomass was highest under the oats, despite the vole damage, with no differences between cutting dates (Table 1.2). The clover mix produced the greatest biomass when averaged across annual crops, due in large part to the red clover with its quick establishment and high spring biomass production.
As was the case at the sprinkler irrigated site, legume treatment did not have any effect on the yield or quality of oat hay at the furrow irrigate site. Similarly, legumes were still too small to contribute to crop yield at the 10 cm cutting height. The contribution of weeds to total yield on a percent basis did not differ between treatments, suggesting that no weed suppression was provided by the mulch crop prior to harvest.
The clover mix was rated higher than white clover in the visual evaluations, indicating superior establishment (Table 1.3). The trefoil did not differ from either the clover mix or the white clover. This is consistent with results for the late-cut oats at the sprinkler irrigated site, which were also harvested at the soft dough stage. Neither legume nor weed biomass differed among legume species. In the sprinkler irrigated field, the clover mix yielded the highest spring biomass across all annual crops. One possible reason for this difference in relative biomass production is the sampling date. Plots at the furrow irrigated site were sampled six days later than those at the sprinkler irrigated site at a time when the legumes were growing rapidly. It is conceivable that the white clover could have grown enough during that time to reduce the differences in biomass among species. Vole damage at the furrow irrigated site was negligible and only occurred in small sections of two of the 40 plots.
In conclusion, co-establishment of perennial leguminous living mulches with corn or oats can eliminate the cost of production loss in the establishment year. This would limit establishment costs to legume seed and planting operations. No yield or quality effects of living mulches were found on either annual crop, and all living mulches were successfully established. Legume establishment was superior with oats due to the earlier harvest of oat hay compared to corn silage. However, these results were obtained in the complete absence of chemical weed control, resulting in high weed populations across conventional and mulch treatments in corn. Weed pressure should be taken into consideration when choosing a site for mulch establishment as weed control options are limited in a living mulch cropping system, particularly in the establishment year.
Objective 2 – Study 1
Based on visual evaluations, there were no differences in legume stand ratings across annual crop strips (Table 2.1). White clover stands consistently received the highest possible rating due to their complete ground cover. This species formed a very thick, stoloniferous mat that proved difficult to cut through with the strip-tiller. These types of thick sod mats are desirable for erosion control and buildup of SOM. Alfalfa followed by red clover received the second and third highest ratings, while treatments that included birdsfoot trefoil received the lowest ratings.
The clovers and alfalfa had the greatest biomass production with trefoil having the least (Table 2.2). Singer et al. (2009) also noted poor persistence of birdsfoot trefoil after the second year when used as a living mulch. There were no differences in ADF values among species, and only trefoil had significantly higher NDF than white clover, red clover and the red clover/birdsfoot trefoil mix. CP was highest for alfalfa and lowest for birdsfoot trefoil, with no differences among the other treatments.
No significant differences in soybean yields were observed as a result of legume treatment. This was not surprising as soybeans are also legumes and are thus capable of meeting most of their own nitrogen needs. While deficiencies can occur if the proper Rhizobium bacteria is not present or some environmental factor limits its activity (Jones, 2003), no deficiency symptoms were observed in the plot area and would not have been expected. Because nitrogen was not a yield-limiting factor, any nitrogen contributed by the legumes would not have significantly impacted soybean yield.
There was, however, the potential for reduced yields due to competition with the perennial. This risk is particularly great in soybeans which accumulate biomass slowly prior to flowering (Pederson and Lauer, 2004). Delayed biomass production along with a later planting date results in much later canopy formation compared to corn. This allowed the mulch more time to recover without competition from the annual. This is why a light application of glyphosate was made mid-season. It seemed to adequately suppress the mulches as those treatments did not differ in yield from the conventional treatment with no living mulch. This finding differed from Pederson et al. (2009) who saw no effect of increasing number of glyphosate applications on soybean yield. In their case, glyphosate applications of 0.75 kg a.e./ha were made at planting and one week later; at planting, one week later and four weeks later; and at planting, one week later, four weeks later and six weeks later. None of these treatments performed as well as when the mulch was completely killed with a combination of tillage and glyphosate. Two key differences between Pederson et al. (2009) and our study were that they used a different mulch crop, kura clover, which is known to be resilient to glyphosate application (Zemenchik et al., 2000), and less aggressive initial suppression.
The mid-season glyphosate application was made six weeks after planting, which was determined based on observation of competition between the soybeans and mulch crops. This situation highlights the advantage, if not the necessity, of employing an herbicide-resistant annual cash crop so as to retain a post-emergence suppression option. The advantage of using an herbicide resistant annual crop has been highlighted numerous times in the literature (Affeldt et al., 2004; Duiker and Hartwig, 2004).
In the corn silage trial, the 252 kg/ha N rate yielded the highest followed by the 168 kg/ha N rate, though this treatment was not different from the yield of the alfalfa treatment (Table 2.3). All of the legume treatments, with the exception of the birdsfoot trefoil-red clover mix, yielded higher than the 84 kg/ha N treatment, indicating that there was at least some yield increase that resulted from nitrogen inputs by the living mulches. The alfalfa, birdsfoot trefoil and white clover treatments, which did not differ from one another in yield, appear to be promising living mulches in terms of their positive effects on corn silage yield.
Treatments containing red clover may have been at a disadvantage in that this was the third year of perennial legume production. While red clover is botanically classified as a perennial, it is known to behave like a biennial (Taylor and Quesenberry, 1996). It is generally accepted that red clover will only be highly productive for two and sometimes three years (Frame et al., 1998). Recommendations for red clover in a mixed pasture involve interseeding every two to three years to maintain the population (Brann et al., 2000). Due to competition and physiological stress from chemical and cultural suppression, similar guidelines would likely need to be followed in a living mulch system. This is further supported when examining legume mortality in the spring of 2010.
Silage quality factors were influenced by legume and fertility treatments (Table 2.3). Crude protein was highest in the 252 kg/ha N treatment and fell as N fertility rate was reduced. The positive correlation between N fertility and CP is well established (Cox and Cherney, 2001; Sheaffer et al., 2006) and would suggest higher levels of plant available N in the alfalfa and white clover treatments. The silage produced in these two treatments had higher levels of CP than the other living mulches and also tended to yield high.
There were fewer differences in fiber concentration among treatments. The lowest fiber contents were found at the two highest N rates, while ADF of the silage produced in the alfalfa treatment did not differ from the 168 kg/ha N rate. The NDF value of the alfalfa treatment was greater than all the other mulches except birdsfoot trefoil. None of the living mulch treatments differed from one another in ADF. These limited differences in fiber contents among mulch treatments are not surprising given that differences in nitrogen fertility have not been as clearly correlated to fiber content as CP. Thus, while N does play some role in carbohydrate partitioning within the plant (Sheaffer et al., 2006), it has a much greater impact on protein levels.
Among the living mulch treatments, alfalfa and white clover had the highest grain yields (Table 2.4). However, all living mulch treatment yields exceeded that of the conventional treatment with the same N fertilization rate, showing the net positive effect of the living mulches. Total corn residue did not differ among mulch treatments. Only the alfalfa and birdsfoot trefoil treatments did not exceed the 84 kg/ha N rate in terms of total residue. No differences were observed in production of individual residue components among mulch treatments.
Birdsfoot trefoil biomass was significantly higher than all other legumes in the fall (Table 2.5). However, biomass yield of all legume species was low. Even the trefoil accounted for only 5% of the biomass yield recorded in the spring prior to suppression. This is far below the desired level of production one would hope to see by the end of the growing season. Zemenchik et al. (2000) observed an increase in growth of kura clover under corn during the ear-fill period. This eventually resulted in 60% ground cover in the winter. To achieve such a level of recovery and subsequent production with the legumes used in this study, a less aggressive suppression regime would have to be adopted.
Corn silage and grain yields relative to nitrogen fertility treatments were plotted to generate nitrogen response curves (Figures 2.1 and 2.2). In both cases, the relationship between N-rate and yield produced a classic nitrogen response curve with the greatest slope occurring between the middle two nitrogen fertility rates. The 84 and 168 kg N/ha rates were then fitted with a linear regression (Figures 2.3 and 2.4) that was used to make an inverse prediction of the nitrogen fertilizer equivalency of each legume treatment. The aforementioned N rates were used because the yield means of all living mulch treatments fell between the means of these two fertility treatments. This prediction cannot be used as a direct quantification of BNF due to other variables introduced by living mulches such as altered soil properties and nutrient competition between crops. Instead, it is meant to measure the net effect of the living mulch in terms of reduction in N fertilizer required. Nitrogen equivalencies of legume treatments are presented in Table 2.5. It should be noted that all legume treatments received 84 kg N/ha as a side-dress application.
The confidence intervals surrounding estimates of nitrogen fertilizer equivalencies were relatively large (Table 2.6). This is the result of making an inverse prediction on a line with a relatively flat slope. The intervals on the y-axis (yield) are exaggerated on the x-axis (N-rate). As a result, no significant differences were found among living mulch treatments.
There was no interaction between legume treatment and annual crop in terms of legume mortality (Table 2.7). The soybean treatment had the least remaining legumes. This was likely due to the mid-season glyphosate application followed by the formation of a thick soybean canopy that outcompeted the perennial legumes for light. Alfalfa and birdsfoot trefoil had the most remaining plants, though even these averaged only about 50% ground cover. White clover had very few remaining plants, and no red clover plants persisted. The mix of birdsfoot trefoil and red clover had only trefoil at this point. Based on these evaluations, only the trefoil following corn and possibly the alfalfa following corn were deemed suitable for continued use as a living mulch without reseeding. Even these stands did not have as much regrowth as desired. Based on these results, it would appear that the broadcast application of glyphosate followed by paraquat was too aggressive to maintain any of these species, particularly the clovers, as a living mulch.
This study demonstrated the potential for leguminous living mulches to reduce N fertilizer inputs to corn. Living mulches tended to increase corn yields (grain and silage) beyond those of conventional corn with the same rate of N fertilization. In soybeans, no yield effects (positive or negative) were observed from living mulch treatments. In this study, all legume residues were minimal and would not likely contribute significantly to the forage quality of grazed corn stover. Legume mortality was high and variable by species. If suppression methods could be determined that allow higher survival of legumes, greater fall grazing benefits may exist. While results suggest that living mulch cropping systems may be a viable alternative under irrigation for producers in the semi-arid West, additional trials are needed. Future research in the region should focus on spring suppression regimes to effectively reduce competition with the annual while maintaining the perennial mulch crop.
Objective 2 – Study 2
An interaction between legume species and suppression treatment occurred for corn grain yield (Table 3.1). No legume treatment effects were found at the lower two rates of glyphosate. At the highest rate of glyphosate, corn yield was lower in white clover mulch than in birdsfoot trefoil mulch and the conventional treatment. The conventional treatment out-yielded the clover mix when treated with paraquat. Suppression treatment had no effect on the white clover and conventional treatments. However, the clover mix yielded lowest in the paraquat treatment. The trefoil yield tended to decline with decreasing glyphosate rate and was low in the paraquat treatment.
Based on visual evaluations prior to the July 6th broadcast glyphosate application, the burn-down treatment appeared to be ineffective at suppressing any of the mulch species. It is likely that the legumes had too much biomass accumulation at the time of spring suppression, resulting in incomplete spray coverage and the loss of only the upper part of the canopy. Recovery of both the clover treatments and birdsfoot trefoil were very rapid, and the corn appeared stunted and chlorotic soon after emergence. The later glyphosate application was effective at not only controlling weed competition, but also the competition from inadequately suppressed legumes. This early season competition could easily explain yield losses in the burn-down treatments for the clover mix and trefoil.
Fall legume biomass was also influenced by the interaction of legume species and herbicide treatment (Table 3.2). Trefoil survival was only apparent in the paraquat treatment, suggesting that trefoil’s tolerance to glyphosate in a living mulch system is still very low in the second year after establishment. However, Boerboom et al. (1990) noted a wide range of glyphosate tolerance among birdsfoot trefoil selections from a recurrent selection breeding program that included ‘Leo’ birdsfoot trefoil. A threefold difference in the rate of glyphosate required to reduce fresh weight by 50% (I50) was found among nine selections previously tested in the field (Boerboom, 1989). The biomass of these selections ranged from 16 to 54% of an untreated control when evaluated as ramets 14 days after treatment with 0.5 kg/ha of glyphosate. Even the two selections of ‘Leo’ tested ranged from 26 to 45%, making them the second and sixth most tolerant among all selections tested. This tolerance was attributed to the specific activity of 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS). Thus, suitability of trefoil for use as a living mulch suppressed with glyphosate would likely differ among cultivars.
No treatment effect was found on fall legume biomass of the clover mix, while white clover biomass was lower at the highest rate of glyphosate and in the burn-down treatment (Table 3.2). The very limited survival of white clover treated with paraquat was surprising as it recovered significantly less than both the clover mix and trefoil in that same suppression treatment. This would not likely be attributed to the mid-season glyphosate application as white clover appears to have much greater glyphosate tolerance compared to birdsfoot trefoil.
An effect of legume treatment on corn grain yield was found at the furrow irrigated site with corn grown in birdsfoot trefoil yielding significantly higher than in both clover treatments (Table 3.3). The effect of legume species on grain yield can likely be related to its inverse relationship with fall biomass production. Legume biomass was significantly greater in the clover treatments (Table 3.4) with trefoil averaging only 3% of the white clover and clover mix biomass. Since competition between the crops is a likely cause of yield reductions, the higher grain yield in the trefoil treatment, which was completely killed, is not surprising.
Both the white clover and clover mix treatments with glyphosate applied at 1.0 kg a.e./ha yielded significantly lower than the control treatment with no living mulch. Again, this was likely due to competition from the clovers which were inadequately suppressed at the lowest application rate. No other treatments differed from the control, indicating that there was no yield loss due to presence of the living mulches.
It should be noted that all treatments at both sites were followed by a mid-season glyphosate application of 1.0 kg a.e./ha, which likely reduced legume recovery below what it would have been with only the pre-plant suppression treatments. This was deemed necessary because of intense weed pressure and not to provide additional legume suppression, but it likely served to reduce treatment effects. Based on visual evaluation, additional mulch suppression would only have been required in the paraquat treatment. A marked reduction in weed control options is a key drawback to living mulch systems. The need for multiple herbicide applications has been well documented (Zimenchik et al., 2000; Affeldt et al., 2004; Duiker and Hartwig 2004; Sawyer et al., 2010) and should be taken into account when selecting a mulch species.
Fall legume biomass in all treatments was below levels that would significantly add to the diet of grazing beef cattle and help to meet their protein needs. White clover hay averages 22% CP, while red clover and birdsfoot trefoil hay average 16% (NRC, 1984). Kura clover hay harvested in October averaged 19% CP over four years (Singer et al., 2010). It is well-established that cattle will graze corn leaves/husks selectively when pastured on corn stover (Lamm and Ward 1981; Fernandez-Rivera and Klopfenstein, 1989). Leaves/husks averaged approximately 4.8% CP when corn was grown with a kura clover living mulch (unpublished data). Based on these percentages and corn leaf/husk biomass averaged across all treatments at the furrow irrigated site, the approximate legume biomass required to raise the CP of the grazed crop aftermath to a given percent can be calculated. To raise CP to 7%, an approximate protein requirement for dry beef cows (NRC, 1996), 623 and 498 kg/ha of kura and white clover, respectively or 830 kg/ha of red clover or trefoil residue would be required. While the levels of clover residue found in this study would reduce the need for supplemental protein, they could not meet total protein requirements for dry beef cattle.
In conclusion, based on results of these studies, there was no clear effect of glyphosate application rate on living mulch recovery, corn grain yield or corn residue biomass. Among legume species, birdsfoot trefoil was the least resilient to glyphosate application, but it appeared to make some recovery after treatment with paraquat. Such limited recovery suggests its poor suitability as a living mulch, but testing with other cultivars may potentially yield different results. Clovers tended to recover more when treated with glyphosate but also reduced corn grain yields compared to trefoil at the furrow irrigated site. Based on this finding, it would appear that even a modest recovery of legumes during the growing season results in some corn yield reduction.
Objective 2 – Study 3
Due to a legume x spray rate x sampling date interaction, the two sampling dates were analyzed separately. The three-way interaction can be attributed to variable rates of recovery by different legume species with white clover recovering the fastest, resulting in no spray rate effect 16 weeks after application. Spray rate effects were present in all other species at both the 8 and 16 week sampling dates. Differences among spray treatments in birdsfoot trefoil varied across sampling dates with the 1.5 kg a.e./ha rate producing the highest biomass at 8 weeks and the 1.0 kg a.e./ha rate yielding the highest at 16 weeks.
Eight weeks after glyphosate application, white clover had the greatest recovery at all rates (Table 4.1). Red clover followed, and alfalfa had the least recovery, with all treatments yielding less than 3% of the untreated control biomass. Birdsfoot trefoil also recovered very little, averaging 5% of the untreated control across all treatments. However, at the 1.5 and 2.5 kg a.e./ha rates, biomass of birdsfoot trefoil did not differ from red clover. The apparent greater recovery of birdsfoot trefoil at some of the higher herbicide rates can be explained by survival of a small number of plants that were randomly distributed within the plot area. This could be due to variable glyphosate tolerance within the population of trefoil plants (Boerboom et al., 1990).
There was a linear decline in biomass of the clover species as spray rate increased (Figures 4.1 and 4.2), with plants recovering more at the lowest application rate than at the two highest rates (Table 4.1). Responses of the two clover species did not significantly differ from one another. No linear relationships were found for trefoil (p=0.1966) or alfalfa (p=0.4619).
An interaction of legume species by spray rate was also present at the 16-week sampling date (Table 4.2). The smallest biomass reductions were found in the clovers with no differences among spray rates for white clover. All treatments were close to 100% of the control indicating full recovery. There was no biomass decline for red clover with increasing spray rate except at the highest rate. Birdsfoot trefoil recovered to about 55% of the control at the lowest rate, but all other treatments were significantly lower.
The response of red clover to increasing glyphosate rate remained linear (Figure 4.3), while white clover had no response (Figure 4.4). Trefoil had a linear response to glyphosate rate (p<0.0001) at 16 weeks, which did not differ from that of red clover. There was still no response found for alfalfa (p=0.1720).
Based on these results, white clover has great potential as a living mulch that can be suppressed with glyphosate. It was extremely resilient with full recovery at all rates after 16 weeks. Red clover also performed well, recovering at the 1.0 and 1.5 kg a.e./ha rates to the same extent as white clover. Birdsfoot trefoil appears to have some tolerance but may not survive higher rates or repeated applications necessary for weed control. However, Boerboom et al. (1990) noted a wide range of glyphosate tolerance among birdsfoot trefoil selections from a recurrent selection breeding program that included ‘Leo’ birdsfoot trefoil. A threefold difference in the rate of glyphosate required to reduce fresh weight by 50% (I50) was found among nine selections previously tested in the field (Boerboom, 1989). The biomass of these selections ranged from 16 to 54% of an untreated control when evaluated as ramets 14 days after treatment with 0.5 kg/ha of glyphosate. The two selections of ‘Leo’ tested ranged from 26 to 45%, making them the second and sixth most tolerant among all selections tested. This variable tolerance was attributed to the specific activity of 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS), with which it was positively correlated. Further, Duiker and Hartwig (2004) noted that birdsfoot trefoil tended to respond less to competition with corn than other legume species tested as living mulches, indicating that it may have the advantage of greater shade tolerance. Thus, other cultivars of birdsfoot trefoil should be tested as they may be better suited for use as living mulches. Alfalfa has little potential since it recovered to only about 16% of the control at the lowest rate after 16 weeks, while at the higher rates, it yielded less than 2% of the control.
Kura clover is also worth noting in that none of the spray treatments differed in actual biomass accumulation at either the 8 or 16 week sampling dates, indicating no yield loss with increasing rates up to 2.5 kg a.e./ha. While there were problems with plant growth in the control that prevented comparisons among other species, the average biomass of kura clover across treatments at 16 weeks was 1,827 kg/ha. White clover averaged 3,736 kg/ha across treatments at the same date. Poor yields of kura clover in the first one to two years are well-documented (Frame, 2005), and seedling vigor of kura clover has been shown to be less than that of white clover (Speer and Allinson, 1984). Difficult establishment is the main deterrent in the use of kura clover as a living mulch and may give preference to white clover, a more vigorous species that appears to share kura’s tolerance for high rates of glyphosate.
Weeds were controlled only by mowing after the first biomass sampling, and there was an effect of both species and spray rate on weed biomass by the time of the second sampling (Table 4.3). The higher two spray rates had significantly greater weed biomass than the lower two. The legume species with the greatest recovery tended to have fewer weeds, with white clover containing less weed biomass than any of the other species. All clover plots had fewer weeds than trefoil and alfalfa. Both the spray rate and species effects can be attributed to an increase in weed biomass as legume biomass decreased. This weed suppressing effect is a commonly cited benefit of living mulches (Enache and Ilnicki, 1990) and is illustrated in Figure 4.5. Combined poor legume recovery and higher weed biomass among alfalfa and trefoil indicate that the clovers are the preferred candidates for a glyphosate-based suppression regime.
When interpreting these findings, one must consider that the use of these species as living mulches will introduce the added stress of competition for light, water and, in some cases, nutrients from the annual crop. Thus, lesser degrees of recovery would be expected than those found in this study.
The effects of both legume species and spray rate were found to be significant in the pot study. All species recovered to a greater degree at the 1.0 kg a.e./ha rate than the 1.5 and 2.0 kg a.e./ha rates in terms of both shoot and root/crown biomass (Tables 4.4 and 4.5). Among legume species, shoot biomass accumulation relative to the untreated control was greatest for kura clover followed by white clover. Alfalfa was again the least resilient, averaging 1% of the untreated control biomass across all rates. In this case, red clover recovered very little and did not differ from alfalfa or trefoil.
While the effect of spray rate on root development was similar to the effect on shoot biomass, there were no differences found in root biomass among species (Table 4.5). This could be a result of the limited rooting area provided in the pots. Upon washing legume roots, it was noted that the control treatments of some of the species, particularly red clover, appeared to be severely root-bound. This limitation to rooting area could help explain the poor performance of red clover in the pot relative to the field.
The results of the pot study confirmed the resilience of white and kura clover relative to the other species tested. The poor performance of red clover must be balanced against its superior recovery in the field. Birdsfoot trefoil recovery relative to other species was better in the pot study. It was superior to that of alfalfa and did not differ from red clover. As in the field study, alfalfa appeared to have very little potential for use as a living mulch managed with glyphosate due to its lack of recovery at all rates.
The effect of spray rate was much more consistent across species in the pot study, most likely due to limitations in root development. Thus, the sharp drop in biomass at the 1.5 kg a.e./ha cannot be used as an indication of appropriate spray rate. More relevant is the consistency between field and pot above ground biomass results (with the exception of red clover) in terms of relative species recovery. Based on results from both studies, white and kura clover show the greatest potential for use as living mulches.
In conclusion, most living mulch suppression regimes include herbicide(s) applied at a sub-lethal rate. This is necessary to reduce competition with the annual crop from both the mulch crop and weeds. Recovery after herbicide application is essential to the persistence of living mulches. White clover was the most tolerant to glyphosate in the field. Although kura clover could not be compared among species, no effects of glyphosate rate were found on recovery, indicating a very high level of tolerance. In the pot study, kura clover had the greatest tolerance followed by white clover. Birdsfoot trefoil and alfalfa recovery was poor in both the field and pot trials. Red clover recovered well in the field, but did not differ from trefoil or alfalfa in the pot study. While recovery is important to the success of living mulches, exceptionally high herbicide tolerance can lead to inadequate suppression and subsequent annual crop yield losses. Based on visual evaluations, none of the white clover treatments had fallen below 80% of the control by the third week after application, which would be insufficient for planting of an annual crop. Thus, while the focus of this study was to determine relative glyphosate tolerance, additional research is needed to test clover species, particularly white clover, as mulches with various annual crops. Adequate suppression may require the inclusion of herbicides other than glyphosate.
Objective 4 –
Matsuda Farms is an organic operation, which eliminated the possibility of using herbicides to suppress the living mulch. At this site in 2009, we harvested the initial spring growth as hay from an established stand of red, white and kura clover. Following haying, we strip tilled down the center of each flood irrigation bed and planted millet for hay. Unfortunately, the clover came back too quickly and competed strongly with the millet, reducing yields. Following the second harvest for hay in late summer, we again strip tilled and planted winter triticale, a more aggressive, cool-season, winter-annual grass.
The triticale, which had been planted the previous fall, was harvested for hay in mid-June of 2010. It proved to be the most successful annual grass tried at the site in terms of its competition with the living mulch (a mix of red, white and kura clover). Its early spring growth highlights the potential of cool-season annuals to successfully compete with established perennial living mulches, particularly in organic systems where suppression options are limited and establishment of the annual is a challenge. In addition to the high triticale yield, the hay still included a fair amount of clover, improving its quality. The field was then strip-tilled and seeded with sudangrass in early July. However, much like the foxtail millet attempted the previous year, it was unable to compete with the clover. Based on these observations, it appears that even with an aggressive summer annual, mowing and strip-tilling alone are insufficient as a means of living mulch suppression.
Since our second producer withdraw from the project, we established a demonstration seeding at the Western Colorado Research Center at Fruita in 2009. This was a co-establishment study in which we first seeded a mix of red, white and kura clover on the flood irrigation beds followed by planting of corn for grain. A strip of granular Harness herbicide was applied down the middle of the irrigation bed with the corn planter at time of seeding to control weeds and keep any clover from establishing in the center of the bed. This approach worked surprisingly well.
In continuation of the co-establishment study conducted at Fruita in 2009, a legume mix of birdsfoot trefoil, red clover and white clover was seeded with corn using a Buffalo brand no-till corn planter. This co-establishment trial was unique in that the planter was modified to seed the corn, broadcast the legume seed and apply a granular herbicide in a single pass. The herbicide, Harness, was applied in a thin strip down the middle of the bed for weed and legume control in the corn row. By eliminating the need for a separate planting operation, growers can reduce the cost of establishing the mulch to the cost of legume seed. Later in the season, strips within the field were treated with glyphosate for weed control and side-dressed with nitrogen, while others were left with no additional herbicide or fertilization. Grain yield and residue data were collected for each treatment. Overall, there was a decrease in fall legume biomass in comparison to the previous year when legumes were planted in a separate operation prior to corn planting. However, additional modifications could be made to increase legume seed to soil contact and possibly increase legume establishment using the corn planter. Treatments of glyphosate without side-dress N, glyphosate with side-dress N and side-dress N without glyphosate yielded 78, 41 and 22 kg/ha of legume biomass, respectively. Legume biomass was greatest when neither side-dress nitrogen nor post emergence glyphosate were applied averaging 163 kg/ha. However, weed pressure was intense under treatments with no glyphosate application, resulting in unacceptable competition with the corn. We believe that this approach holds promise for establishing the living mulch, but it still needs some slight modifications followed by further field testing.
Affeldt, R.P., K.A. Albrecht, C.M. Boerboom, and E.J. Bures. 2004. Integrating herbicide-resistant corn technology in a kura clover living mulch system. Agron. J. 96:247-251.
Albrecht, K.A., T. Ochsner, and B. Berkevich. 2009. Farming for nitrogen: Intercropping corn and kura clover. p. 163-166. In Proc. 2009 Wisconsin crop management conference, Madison, WI, 13-15 Jan. 2009.
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Boerboom, C. M. 1989. Selection and characterization of glyphosate tolerance in birdsfoot trefoil (Lotus corniculatus). PhD. Thesis, Univ. Minnesota. 67 pp.
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Enache, A.J. and R.D. Ilnicki. 1990. Weed control by subterranean clover (Trifolium subterranium) used as a living mulch. Weed Technol. 4:534-538.
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Sheaffer, C.C., J.L. Halgerson, and H.G. Jung. 2006. Hybrid and N fertilization affect corn silage yield and quality. J. Agronomy and Crop Science. 192:278-283.
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Results from this study suggest that living mulch cropping systems may be a viable alternative under irrigation for producers in the western U.S. Two of the main benefits observed in this study include reduced soil erosion, especially under furrow irrigation, and reduced need for nitrogen fertilization due to symbiotic N-fixation by the legumes. We were disappointed that the legumes did not recover enough in the fall to add much to biomass for fall grazing of the stover/mulch combination.
Mulch crops can be successfully co-established with corn or oats, thereby reducing the cost of establishment, which was one concern producers had with adopting this system. White clover shows potential as a living mulch due to its positive effects on corn grain and silage yields when adequately suppressed. It also has high glyphosate tolerance. Kura clover also has high glyphosate tolerance, but it is slow and difficult to establish. Seed of kura clover has also been difficult to find, as well as expensive, in recent years compared to other legumes. Leguminous living mulches can reduce nitrogen fertilizer needs, but adequately suppressing the mulch to minimize annual crop yield losses while maintaining the perennial legume stand remains a challenge. A combination of strip-tilling and herbicide application is the most effective suppression strategy. However, finding the appropriate herbicide and rate has proved difficult.
From our observations during the growing season, it appears that the burndown herbicides tested allowed for rapid recovery of the legumes, which resulted in competition between the annual crop and the mulch. When using glyphosate, choosing the appropriate rate proved difficult. Too light and the mulch competed strongly with the annual crop. Too heavy of a rate resulted in the mulch being killed. To compound things, the rate of glyphosate that worked well at one point either worked too well or not at all the next time. There are a myriad of factors that can influence herbicide efficacy including air and soil temperature, soil moisture, amount of growth on the perennial mulch at time of application and how long the perennial mulch has been established. In our opinion, choosing the appropriate herbicide and rate remains as one of the biggest obstacles to be overcome before producers will embrace this system.
Further, field experiences confirmed the necessity of growing an herbicide-resistant annual crop. A mid-summer glyphosate application for weed suppression is often necessary. Without this option, yield losses through competition with the mulch, weeds or both can be substantial.
Educational & Outreach Activities
Beahm, A.T. 2011. Establishment and maintenance of leguminous living mulches for irrigated systems in the semi-arid West. M.S. Thesis, Colorado State University, Fort Collins, CO.
Beahm, A.T., J.E. Brummer, C.H. Pearson, and N.C. Hansen. 2010. Living mulches for irrigated corn and soybeans in the semi-arid West. Abstr. 122-3, ASA-CSSA-SSSA Annu. Meeting, Long Beach, CA.
Beahm, A.T., J.E. Brummer, C.H. Pearson, and N.C. Hansen. 2011. Living mulches for irrigated corn and soybeans in the semi-arid West. In: Abstr. Book, Institute for Livestock and the Environment 4th Ann. Stakeholder Summit, Nov. 17, 2011, Fort Collins, CO.
Pearson, C.H., J.E. Brummer, and A.T. Beahm. 2010. Co-establishment of legumes and corn in a living mulch cropping system under furrow irrigation, p. 10-19. In: R. Zimmerman (ed.), Western Colorado Research Center 2009 Annual Report, Colo. Agri. Exp. Sta. Tech. Rep. TR10-07, Fort Collins, CO.
Two field tours were held in 2009; one at Fruita, CO with 25 in attendance and one at Fort Collins, CO with 14 in attendance. One presentation entitled “Potential Benefits of Living Mulch Cropping Systems” was given at an irrigation workshop for producers with 17 in attendance.
In 2010, a poster entitled “Living Mulches for Irrigated Corn and Soybeans in the Semi-arid West” was presented at the combined American Society of Agronomy (ASA), Crop Science Society of America (CSSA), and Soil Science Society of America (SSSA) International Annual Meetings in Long Beach, California. Another poster entitled “Corn Production Using Living Mulch Cropping Systems” was displayed at a media day at the Western Colorado Research Center at Fruita, CO in which local journalists were invited to learn about current agricultural research in western Colorado. A seminar was also presented to about 40 students and faculty in the Soil and Crop Sciences Department at Colorado State University.
In 2011, another seminar entitled “Establishment and Maintenance of Leguminous Living Mulches for Irrigated Systems in the Semi-Arid West” was presented to about 20 students and faculty in the Soil and Crop Sciences Department at Colorado State University. A poster entitled “Living Mulches for Irrigated Corn and Soybeans in the Semi-arid West” was presented at the Institute for Livestock and the Environment 4th Annual Stakeholder Summit with about 35 faculty, students and representatives from various agricultural industries in attendance. In August, personnel from the Western Colorado Research Center at Fruita spent an evening at the local farmers market handing out flyers and discussing ongoing agricultural research which included information on the living mulch project. Several hundred individuals came by the booth. The final outreach activity for 2011 was a field tour for 30 private individuals held in September at the Western Colorado Research Center at Fruita in which one of the stops included a visit and discussion of a corn/living mulch co-establishment plot.
In 2012, a presentation entitled “Advantages and Disadvantages of Living Mulches in Cropping Systems” was presented at a Natural Resources Conservation Service sponsored Soil Health Conference in Montrose, CO to about 100 attendees representing producers, NRCS field personnel and other agencies.
A complete economic analysis was not conducted for this study. Due to the short-term nature of this project and the difficulty in adequately suppressing the living mulch without killing it, we were not able to gather enough data to confidently address this aspect. With that said, we will offer a few insights into potential returns and risks associated with adopting this type of cropping system.
We addressed the concern of producers about the need to take land out of production (i.e. loss of a cash crop) during the year when the living mulch was established. We are confident that the living mulch can be successfully co-established with several types of annual cash crops.
One of the biggest risks associated with this system is failing to adequately suppress the perennial mulch which results in a loss of production from the annual cash crop. If this happens, then any potential gains from say nitrogen fixation by the legumes and reduced fertilizer inputs will be nullified. Basically, the perennial mulch acts much like weeds do in the system, competing for water and nutrients. So it becomes a real balancing act to suppress the living mulch enough so it does not compete with the annual crop without killing it. This may end up being too much risk for most producers to ever consider adopting this system.
On the positive side, we did find that the perennial legumes provided at least some nitrogen contribution (20 to 70 kg/ha) to the annual crop, thereby partially offsetting the need to apply as much nitrogen fertilizer. At today’s nitrogen prices, this equates to $26 to $92/ha, which is a significant savings.
Applying costs and benefits to such factors as reduced soil erosion, carbon storage and increased soil organic matter becomes more difficult, especially in a short-term study like this one. It often takes many years to realize the full benefits associated with these types of changes.
To our knowledge, there have not been any producers that have actually adopted this system in Colorado or any of the other western states. Based on feedback from producers following several of the outreach presentations, there is a high level of interest from a select few that want to look into it in more detail. They are currently weighing the pros and cons and studying their options. Hopefully, a few will give it a try on a small scale. Based on our experience, there will be a steep learning curve for anyone who implements a living mulch cropping system. We believe that the benefits outweigh the risks for those willing to invest in the higher level of management that will be required to make the system successful.
Areas needing additional study
The biggest factor limiting farmer adoption of this system is the uncertainty surrounding management of sub-lethal rates of herbicide that are needed to suppress the living mulch. Because of the many factors that can affect herbicide efficacy, it is difficult to provide farmers with a cookbook recipe to follow. Too high of a rate and you kill the living mulch, and too low of a rate and the living mulch competes with the annual cash crop reducing yields. In our experience, most farmers want that cookbook recipe, and unless we can give it to them, they will be reluctant to adopt this system. Additional on-farm trials working with very progressive producers that have the management skills necessary to make these types of system successful will be the most effective way to demonstrate the benefits to others.