Integrating Perennial Living Mulches into Irrigated Cropping Systems

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.

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• Obj. 2 – Study 2 Tables
• Obj. 2 – Study 3 Figures
• Obj. 2 – Study 1 Tables
• Obj. 2 – Study 1 Figures
• Obj. 1 Tables
• Obj. 2 – Study 3 Tables