Corn (Zea mays L.) is a productive and popular forage crop that can exacerbate soil loss, surface water runoff, and nonpoint source nutrient pollution from agricultural fields.
During 2010 and 2011 we compared the effects of kura clover (Trifolium ambiguum M. Bieb.) living mulch and winter rye (Secale cereale L.) in corn silage production on runoff, soil physical properties and organic matter, and forage yields. At the University of Wisconsin Lancaster Agricultural Research Station 8 km west of Lancaster, WI we simulated short, heavy rainstorms on loess soils with 8 to 15% slopes.
Kura clover living mulch reduced water runoff by 50%, soil loss by 77%, and P and N losses by 80% relative to monocrop corn. Rye reduced water runoff by 67%, soil loss by 81%, P loss by 94%, and N loss by 83% when planted after corn silage harvest. When rye was planted following corn silage in kura clover living mulch, water runoff was reduced by 68%, soil loss by 77%, P loss by 94%, and N loss by 84% relative to monocrop corn. Dissolved reactive P, NH4-N, and NO3-N losses in runoff were often, but not always, higher in monocrop corn.
Most of the differences between cropping treatments were due to differences in runoff amount rather than concentrations. Higher ground cover, soil aggregate stability, and soil organic matter, as well as soil surface disturbance from rye planting, were associated with improved infiltration and reduced soil and nutrient losses in the cover cropped treatments.
When grown in kura clover living mulch, both corn and rye had lower yields but this was offset by lower fertilizer requirements and improved farmland and environmental function and quality.
Corn silage is one of the most common forage crops used in Wisconsin and throughout the United States (NASS, 2011). The removal of almost all corn residue, as occurs in silage production, can lead to much larger amounts of surface water runoff, soil erosion, and nutrient pollution than is seen when stover is left in the field (Blanco-Canqui and Lal, 2009; Grande et al., 2005a; Grande et al., 2005b; Lal, 2009; Weinhold and Gilley, 2010). While silage is a relatively minor use of corn (NASS, 2011), the possibility of corn stover based bioenergy products may leave many more corn fields bare and vulnerable during the fall and spring. Additionally, the removal of stover leads to lower organic matter and poor soil structure, reducing agricultural productivity in the long run (Blanco-Canqui and Lal, 2009, Fageria et al., 2005).
Soil, nutrient, and water losses exacerbated by bare soil create need for additional fertilizer and irrigation inputs. Agricultural runoff also contributes to widespread nonpoint source pollution and surface water eutrophication seen throughout the United States (Carpenter et al., 1998). The products made from whole-plant corn, whether silage, biofuel, or bio-based materials, are obviously quite attractive to many farmers and useful to society at large. However, they must be paired with agronomic practices designed to mitigate the long-term possibility of farmland and environmental degradation.
Cover crops reduce surface runoff and soil loss from agricultural fields by improving soil structural stability and infiltration, holding soil particles with their roots, and blocking raindrop impacts with their leaves (Berg et al., 1988; Dabney, 1998; Sharpley and Smith, 1991; Zuazo and Pleguezuelo, 2008). Despite these, and other, agronomic benefits, cover crops are little used in the field crop production occupying the great majority of American farmland (Clark, 2007; Hartwig and Ammon, 2002; Lubowski et al., 2006; Singer 2008).
Midwestern, field-crop farmers surveyed by Singer et al. (2007) identified the cost of cover cropping as a limiting factor in adoption. Government action could greatly increase the use of cover crops but the regulation of agricultural runoff has been controversial, unevenly implemented, and largely ineffective in alleviating the problem of nonpoint source pollution nationally (Graham et al., 2011; Scanlan, 2011). The development of economically viable, perennial living mulches to supplement more popular, annual covers could increase the use of cover crops, even in the absence of more effective cost-sharing programs (Clark, 2007; Singer 2008). Though perennial living mulches must be carefully managed to prevent resource competition with cash crops, they do not need regular re-establishment. The maintenance of year-round ground cover can provide agronomic and environmental benefits during periods when annual covers would be absent.
Particularly promising for corn production in the northern corn belt is the use of kura clover living mulch. In this system, corn yields were only slightly reduced using conventional hybrids under favorable weather conditions and not reduced when using herbicide resistant corn hybrids and suppressing the clover during the growing season (Affeldt et al., 2004; Zemenchik et al., 2000). Offsetting the time and opportunity costs of kura clover living mulch management, it substantially decreases N fertilizer needs for corn production and reduces NO3-N leaching to groundwater (Berkevich, 2008; Ochsner et al., 2010). Furthermore, kura clover living mulch was associated with higher natural enemy populations leading to increased predation of European corn borer (Ostrinia nubilalis Hübner), and was among the best species for weed suppression (Prasifka et al., 2006; Singer et al., 2009). Kura clover has also been successfully managed for high quality forage production as a monocrop in conjunction with winter and spring small grains and in perennial mixtures with forage grasses (Contreras-Govea and Albrecht, 2005; Contreras-Govea et al., 2006; Kazula, in review; Kim and Albrecht, 2011). It is persistent and productive in northern climates and is resistant to many diseases that affect other clovers (Trifolium spp.), possibly providing permanent ground cover (Cuomo et al., 2003; Sheaffer and Marten, 1991; Taylor, 2008).
Despite its potential, kura clover living mulch has not been adopted by farmers, possibly due to the complexity of its management and the agronomic restrictions on cash crop options, as well as the lack of widespread knowledge of the system. Kura clover is slow to establish but corn or other companion crops [e.g. oats (Avena sativa L.) or birdsfoot trefoil (Lotus corniculatus L.)] may be grown during the establishment year (Jusoh, 2010; Seguin et al., 1999; Sheaffer and Marten, 1991). Unfortunately, kura clover stands can be negatively affected by companion crops. Once established, the use of kura clover as a living mulch presents additional management issues. Competition for water and mineral nutrients can reduce corn yields and neither herbicide nor mechanical suppression of kura clover living mulch has proved to unfailingly eliminate yield losses (Bard, 2009; Sawyer et al., 2010; Zemenchik et al., 2000). Greater suppression of the living mulch assures corn yields during the same year but could damage the long term persistence of the kura clover. Furthermore, kura clover living mulch has been problematic in soybean production [Glycine max (L.) Merr.], for either seed or forage (Pedersen et al., 2009). As a result, kura clover living mulch has limited potential in the common corn-soybean rotation or in conjunction with other leguminous seed crops.
Unlike kura clover living mulch, winter rye is in use on farms as a cover crop following corn (Clark, 2007; Stute et al., 2007). Both kura clover and winter rye have been studied in a wide variety of contexts and directly compared in terms of forage yield and quality (Kazula, in review) as well as in their impact on soil properties (Jokela et al., 2009) as cover crops in corn silage systems. Additionally, winter rye has reduced water runoff and erosion in corn silage and soybean production (Kaspar et al., 2001; Laloy and Bielders, 2010) and preliminary research by Eleki (2003) found a reduction in sediment and phosphorus losses with the implementation of a kura clover living mulch relative to conventionally tilled corn silage. To build on this research, our goal was to directly compare kura clover living mulch and winter rye to monocrop corn effects on erosion and nutrient runoff.
Our short-term objectives, to document the environmental effects of kura clover living mulch and winter rye in no-till corn silage on surface runoff was was completed during 2011. The results were published as a Master’s thesis, presented as a seminar in Madison at the University of Wisconsin campus for an audience of professors, research staff, and graduate students, and shared with research station staff in southwestern Wisconsin.
The research is being submitted as a poster presentation at the 2012 Agronomy Society of America conference and will be submitted for peer-review publication as well.
Research in forage production systems incorporation kura clover living mulch is ongoing at the University of Wisconsin and accessible cropping systems will be promoted through direct communication with extension agents and as a brochure detailing both the agronomic management and environmental benefits of kura clover living mulch.
This research was conducted on loess derived Fayette silt loam (fine-silty, mixed, superactive, mesic Typic Hapludalf) with slopes of 8 to 15% at the University of Wisconsin Lancaster Agricultural Research Station 8 km west of Lancaster, WI (42°50′ N, 90°48′ W, 300 m above sea level). The soil was alkaline (pH of 7.6) with low potassium (85 mg kg-1) and phosphorus (13 mg kg-1) levels for corn silage production (Laboski et al., 2006) but soil nutrient levels were corrected with starter fertilizer. Wishmeier and Smith (1978) computed this soil to have a moderately high erodibility K-value of 0.38.
Four treatments were planted as 15 by 18-m plots in a randomized complete block design with four replications: 1) monocrop corn silage (MC), 2) corn silage with winter rye cover crop (R), 3) corn silage grown in kura clover living mulch (K), and 4) corn silage grown in kura clover living mulch with winter rye (KR), and subjected to simulated rainstorms during 2010 and 2011.
KTA202 kura clover was established in 2007 over the entire experimental area, which had been previously cropped in alfalfa, and harvested three times during 2007 and 2008 while it became well established. In 2009, kura clover was killed in the MC and R treatments and sorghum x sudan grass [Sorghum bicolor (L.) Moench] was grown to create kura clover free plots. Kura clover continued to grow in the K and KR treatments. Kura clover was harvested four times and sorghum x sudan grass was harvested three times in 2009 with the last harvest in October.
All treatments were sprayed with 833 g a.i. ha-1 of glyphosate (N-(phosphonomethyl)glycine) and 72 g a.i. ha-1 dicamba (diglycolamine salt of 3,6-dichloro-o-anisic acid) on 14 April 2010, to suppress the kura clover and kill weeds throughout the experiment before corn planting. This was a higher rate of dicamba than has often been used for kura clover living mulch suppression (Affeldt et al., 2004) because the kura clover was 15 cm tall and previous experience had demonstrated that a higher level of suppression would be required. On 3 May 2010, glyphosate resistant, hybrid corn ‘DeKalb DKC45-79’ was planted using a White no-till corn planter (White Tractor Company, Burgaw, NC) in 76 cm rows at a rate of 86,000 seeds ha-1 (Table 1). Starter fertilizer with 21 kg ha-1 N, 35 kg ha-1 P, and 86 kg ha-1 potassium was applied with the corn seed. Twenty five-cm wide strips of clover in K and KR treatments, centered on the rows of corn, were then killed using 278 g a.i. ha-1 dicamba, 54 g a.i. ha-1 flumetsulam (N-(2,6-difluorophenyl)-5-methyl-1,2,4-triazolo-[1,5a]-pyrimidine-2-sulfonamide), and 145 g a.i. ha-1 clopyralid (3,6-dichloro-2-pyridinecarboxylic acid).
When corn was about 30 cm tall and in the V7 growth stage, on 9 June 2010, 180 kg ha-1 N, in the form of NH4NO3, was sidedressed on MC and R treatments while 45 kg ha-1 N was applied to K and KR treatments. The applications were meant to meet the nitrogen needs of the corn silage crop and prevent nitrogen from functioning as a yield-limiting factor. The next day 833 g a.i. ha-1 of glyphosate was sprayed onto all treatments to control the kura clover living mulch and weed growth throughout the experiment.
Approximately 2 weeks after the corn silage harvest, on 10 September 2010, ‘Spooner’ winter rye was planted in 19 cm rows at a rate of 135 kg ha-1 using a Krause no-till drill (Kuhn Krause, Inc., Hutchinson, KS) in R and KR treatments. No additional fertilizer was applied to the rye crop. The next spring, on 21 April 2011, bare plots (MC) were sprayed with 833 g a.i. ha-1 of glyphosate and 38 g a.i. ha-1 dicamba to prevent weed growth.
Rainfall simulations were conducted between 22 and 27 April, on 1 and 2 June, and on 8 and 9 September 2010, on monocrop corn and kura clover living mulch treatments. In fall 2010, winter rye was planted into half of the plots creating the R and KR treatments. The last two simulations (between 18 and 22 October 2010 and 2 to 4 May 2011) were performed on all four treatments. An undisturbed section of each plot was used for each rainfall simulation.
The simulations were timed in order to capture runoff events when traditional corn silage systems are at their most vulnerable (least ground cover) and when the rainfall simulator could be effectively assembled, operated, and moved throughout the plot area (relatively short vegetation). High intensity, short storm events occur most often in southwestern Wisconsin during the summer and the summer months are generally wetter overall (Angel and Huff, 1995; NCDC, 2011) (Table 2). However, corn fields are much less likely to produce runoff events during these storms due to the canopy cover provided by the corn itself and low antecedent soil moisture (Bui and Box, 1992; Stuntebeck et al., 2011). During the spring and fall, the simulations were timed in order to represent as fully as possible the impacts of the cover crops.
The first simulation was performed after kura clover suppression but before corn planting. The second simulation was conducted when the corn was at approximately V6 stage and just before side-dressed N application. The third simulation was immediately following corn silage harvest and preceding winter rye planting. With all four cover cropping treatments present in the plot area, two final simulations were performed. The fourth simulation was conducted after the winter rye had established and the kura clover had begun to recover. The final simulation was performed before rye silage harvest or hypothetical corn planting. The management preceding the final simulation could represent either a forage rotation in which corn silage is followed by a rye forage crop or continuous corn production in which rye cover crop would be killed using herbicide. At the time of the last simulation, rye was 20 to 25 cm tall.
Before the rainfall simulations, ground cover, slope, and soil moisture were measured in or adjacent to each simulation subplot. Ground cover was measured using 10 points from each of four 1-m transects across the subplot. Slope was calculated by averaging measurements from both sides of the subplot. Volumetric soil moisture was measured six times using an ML2 theta probe and Delta T HH2 moisture meter (Delta-T Devices, Cambridge UK) around the subplot and averaged. Soil moisture was measured approximately 2 m from the actual site of the simulation in order to avoid areas with substantial foot traffic which could have affected the resulting values. A composite sample of 10 1.9-cm cores of the top 5 cm of soil was collected from the area surrounding each subplot to test for Bray-1 extractable P, NH4-N, and NO3-N. The well water used in the rainfall simulation was analyzed for dissolved elemental (K, P, Ca, Mg etc.), NH4-N, and NO3-N concentrations and pH (Table 3).
The rainfall simulations followed the methods of the National Phosphorus Project (NPP) using a Tlaloc 200 Rainfall Simulator (Joern’s Inc., West Lafayette, IN) with a TeeJet 1/2HH-SS50WSQ nozzle (Spraying Systems Co., Wheaton, IL) positioned 305 cm above the soil surface (Humphry et al., 2002) to simulate a 1 hour storm with 70 mm of rainfall. This rate corresponded to a 50-year storm event in southwestern Wisconsin (Huff and Angel, 1992). The simulations were not performed during natural rainfall events or high winds so that the simulated rainfall events would be of uniform intensity and drop velocity. For each simulation, a 2 by 2-m steel frame was hammered approximately 7.5 cm into the ground, leaving the downslope side flush with the soil. The other three sides remained about 7.5 cm out of the soil. In situ soil and silicon caulk were used to fill cracks when they developed along the inside edge of the frames during installation. The downslope side of the frame was fitted with a PVC trough leading out of the subplot into a small pail, pumped into a barrel, and weighed. The time to continuous runoff from the trough, rather than drips, was recorded using a stopwatch during each simulation. At the end of the rainfall, the runoff was weighed and a well-mixed, 1-L, unfiltered sample was collected to measure suspended sediment, total N, and total P. Another 60-mL sample was passed through a 45µm filter (Millipore Corp., Billerico, MA) to characterize dissolved N and P species. When there was not enough runoff for both samples, only the unfiltered sample was collected. This occurred once in June of 2010 in the K treatment and once in May of 2011 in the KR treatment.
Unfiltered runoff samples were analyzed for suspended sediment, total P, and total N. A subsample was dried at 105°C to determine the suspended sediment concentration of the runoff (Peters et al., 2003; Wolf et al., 1997). Total P was determined using perchlorate digestion (EPA 600) while total N was determined using digestion with sulfuric and salicylic acids (Bremmer, 1965). Dissolved reactive phosphorus, NO3-N, and NH4-N were measured in filtered runoff, and in the well water, using perchlorate digestion (EPA 600) for dissolved reactive P and automated colorimetry with a KCl extraction for both NO3-N (EPA 353.2) and NH4-N (EPA 350.1).
Soil samples collected at the time of the simulations were analyzed for soil NO3-N and NH4-N using flow injection analysis and a KCl extract (Ruzicka, 1983). Plant available P was determined with a Bray P1 extract (Bray and Kurtz, 1945).
A 10-m row of corn was harvested for silage at approximately 50% milk stage from MC and K treatments on 30 Aug. 2010. The next spring, a 1 by 1-m (5 row) section of rye was harvested during the beginning of inflorescence emergence [10.1 Feekes scale (Large, 1954)] from R and KR treatments on 18 May 2011. For both harvests, the forage was weighed to measure total yield and a subsample was dried at 60° C to determine dry matter percentage and dry matter yield and then used to analyze forage quality.
Dry corn and rye silage samples were ground through a 1-mm screen (Wiley Mill; Thomas Scientific, Swedesboro, NJ). Nitrogen concentration was determined using rapid combustion of nitrogen in a Leco FP-528 (Leco Corp., St. Joseph, MI) and crude protein calculated by multiplying the nitrogen content by 6.25. The forages were also analyzed for neutral detergent fiber content (NDF) by extraction in a neutral detergent solution with sodium sulfite and ?-amylase following the procedures of ANKOM Technology Corp. (Fairport, NY). Duplicate subsamples of forage from each plot were incubated in a mixture of filtered, bovine rumen fluid and mineral buffer before undergoing neutral detergent extraction to determine in vitro true digestibility (IVTD) using the Daisy II system of ANKOM Technology Corp.
Soil samples were collected following the completion of simulations and rye harvest, on 18 and 19 May 2011. In each plot, 10 1.9-cm soil cores were collected at two depths, 0 to 5 and 5 to 15 cm, mixed to form composite samples, and analyzed for organic matter content by weight loss on ignition (Schulte and Hopkins, 1996) and wet aggregate stability using wet-sieving (Cambardella and Elliot, 1993). Macro-aggregates were separated into three size classes during wet sieving, small (0.25 to 2 mm), large (2 to 8 mm), and all (0.25 to 8 mm), and each class was dried and weighed separately.
Penetration resistance and volumetric soil moisture were also measured on 18 and 19 May. Penetration resistance was measured using a Field Scout SC900 soil compaction meter (Spectrum Technologies, Inc., Plainfield, IL) with a 1.28 cm wide cone pushed manually into the soil at a rate not exceeding 5 cm s-1. Four penetration resistance measurements were taken in each plot from the surface down to 30 cm; two were taken from interrows with recent planter traffic and two from relatively traffic free interrows. Volumetric moisture was determined with a ML2 theta probe and Delta T HH2 moisture meter. Soil moisture was measured twice per plot for three depth zones: 0 to 10, 10 to 20, and 20 to 30 cm.
Each of the five sets of simulations, as well as soil properties, were analyzed independently. In PROC MIXED (SAS inst., 2006) analysis, blocking was treated as a random variable with treatment as a fixed effect. The runoff, soil properties, and forage quality response variables were ranked using PROC RANKS (SAS inst., 2006) to account for non-normal residuals. Analysis of significance, at P = 0.05, was performed on the ranks using Fisher’s LSD protected by ANOVA, also at P = 0.05, and the PDMIX800 macro (Saxton, 1998). PROC REG (SAS Inst., 2006) was used to perform linear regression on the correlation of ground cover and water runoff, suspended sediment concentration, and soil loss.
- Table 2. Mean monthly precipitation and temperature at the Lancaster, WI Agricultural Research Station from January 2009 to June 2011 and 1981-2010 normals.
- Table 3. Mean elemental concentrations and pH found in well water used in rainfall simulations.
- Table 1. Timeline of crop management, rainfall simulations, and soil assessment.
Total dry matter yield for both corn and rye crops were lower in treatments with kura clover living mulch (K and KR) than those without (MC and R) (Table 4). Dry matter content of the harvested corn silage (above-ground biomass) was lower in K than MC, likely due to a slight delay in maturation in K, as is typical for corn grown in kura clover living mulch (Albrecht, unpublished data). Rye forage dry matter yield was lower in KR than R but neither dry matter percentage nor stage of maturity was different. In plots with both kura clover and rye (KR), kura clover accounted for little (approximately 1%) of the total biomass harvested for forage. In both forage crops, crude protein, neutral detergent fiber, and in vitro true digestibility were not different between crops grown with and without kura clover living mulch.
Organic matter concentration was greater in the surface (0 to 5 cm) soil than at greater depths (5 to 15 cm) (Table 5). Organic matter was highest in KR and K, followed by WR and MC treatments. Total (0.25 to 8 mm) and small (0.25 to 2 mm) macro-aggregate stability was not different (p > 0.07) between treatments. There were more stable, large macro-aggregates (2 to 8 mm) in K and KR than MC and R treatments. While total macro-aggregate stability did not vary (p > 0.15) with depth, stable, large macro-aggregates were more common between 5 and 15 cm and stable, small macro-aggregates were more common between 0 and 5 cm.
Wheel traffic from planting did not result in differences in penetration resistance (p > 0.05) (Table 6). At the surface (0 to 2.5 cm), MC had higher penetration resistance than other treatments. Between 7.5 and 30 cm MC had lower resistance compared to any cover cropped treatments. Despite the differences in management, all treatments received the same amount of traffic, from planting (sorghum x sudangrass, corn, and rye), harvesting, and rainfall simulations. At a depth of 5 cm, no treatment differences were found (p > 0.07). The MC treatment had higher volumetric water content at all depths (0 to 10, 10 to 20, and 20 to 30 cm) (Table 7), although not always significantly (p < 0.05) so compared to all other treatments. At most depths, there were no differences between R, K, and KR treatments (p > 0.05) and when they were (0, 7.5, and 15 cm) there was not a consistent ranking of the cover crop treatments.
Nutrient levels (P, NH4-N, and NO3-N) in well water used for rainfall simulations and plot slope did not affect runoff nutrient concentration based on analysis of covariance. Soil Bray-1 P did not differ between any treatments (p > 0.2) at the time of any simulations (Table 8). Soil NO3-N did not differ between MC and K treatments during any simulation. In October 2010, soil NO3-N was lowest in KR and R treatments with MC higher than them. Kura clover living mulch was intermediate and not different from any other treatment. Both kura clover and winter rye have been shown to reduce soil NO3-N (Oschner et al., 2010; Krueger at al., 2001) but only winter rye reduced soil NO3-N in this experiment. In May 2011, soil NO3-N was low compared to other simulation dates and lowest in R but other treatments were not different from each other. In June 2010, soil NH4-N was lower in the MC than in the K treatment but no differences occured among treatments at other simulation dates.
Volumetric soil moisture was never different between MC and K or between R and KR treatments (Table 8). In September 2010, soil moisture was lower in treatments with rye (R and KR) than in those without (MC and K), possibly because of the planting operation, exposing subsurface soils to the atmosphere, and water use by the growing rye plants. In May 2011, treatments with rye had higher soil moisture than the others. While these differences were significant, they were never more than 4% and probably had little effect on the rainfall simulations.
Ground cover varied, often greatly, among treatments in all simulations. Monocrop corn ground cover was lower than any other treatment during all simulations. The largest difference in ground cover between treatments was seen in April of 2010 when it was much higher in K than in MC. In September 2010, MC ground cover was fairly high (27%) and K ground cover was relatively low (39%). While MC also had fairly high ground cover (27%) in May 2010, the other treatments all had much higher ground cover (> 73%). In October 2010, KR had the highest ground cover while R and K were intermediate.
In April of 2010, water runoff; soil loss; total P and N losses; and dissolved reactive P, NH4-N, and NO3-N losses were lower in K than in MC (Table 9). While the suspended sediment and the overall P and N concentration of the runoff were lower in the K treatment, levels of dissolved nutrients in the runoff were not different. The lower dissolved nutrient losses in K were the result of lower total runoff while total P and N and soil losses were the result of both less runoff amount and lower concentration in the runoff from K. Time to runoff was not different in MC than in K.
In June and September 2010, water runoff, soil erosion, and total P and N losses were lower in K than MC but dissolved nutrient losses were not different. The difference in ground cover between K and MC was not as large as in April but still significant. The suspended sediment and nutrient concentration of the runoff samples were not different in June, except for dissolved NH4-N where the significant differences were very small, but the larger volume of runoff in MC resulted in the differences in losses. In September, suspended sediment and total P and N concentrations were lower in K than MC, compounding the differences in runoff volume on soil and total P and N losses while dissolved nutrient concentrations were not different. Time to runoff was shorter in MC during June but not different in September. While the relative treatment effects of the June and September simulations are similar, both treatments had higher amounts of runoff and lost more soil, N, and P in September than in June 2010.
Time to runoff played a large role in October 2010, with rye treatments having longer times to runoff and lower runoff amounts. Kura clover with rye had much less water runoff than any of the other treatments. Rye following monocrop corn silage had the second lowest runoff volume. While K had considerably more runoff than either rye treatment, it was still lower than the amount resulting from MC. Suspended sediment concentration was higher in MC and KR than R and K while total P concentration was higher in MC than any other treatment. No differences were seen in total N or dissolved reactive P concentrations and the differences in dissolved NH4-N were small. The same pattern seen in the amount of runoff occured in soil, total P, total N, and dissolved NH4-N losses. While there was no difference between K and MC or between R and KR treatments in dissolved reactive P losses, the treatments with rye lost less dissolved reactive P than those without. Essentially no dissolved NO3-N was lost from KR while MC lost more dissolved NO3-N than either R or K even though R had a much higher dissolved NO3-N concentration than MC, or any other treatment. During this set of simulations, the large differences in the amount of runoff from the treatments again overwhelmed the effects of differences in concentration.
In May 2011, MC produced more runoff, lost more total N, total P and dissolved NH4-N, and had a shorter time to runoff than any of the cover cropped treatments (Table 9). Corn silage followed by winter rye and K treatments in turn lost less water, total NO3-N, and dissolved NH4-N than KR. Total P and N concentrations were also higher in MC than in cover crop treatments, exaggerating the differences seen in the amount of runoff while there were no treatment differences in suspended sediment or dissolved nutrient concentrations. Although MC lost the largest amount of soil, it was not different from the amount lost by KR. Of the four treatments, R and K lost the least soil but they were not different than KR either. Dissolved reactive P losses were highest in MC, but not higher than in KR. Corn silage with kura clover living mulch lost the smallest amount of dissolved reactive P but not less then R, which was not less than the amount lost by KR. No dissolved NO3-N was measurable in the runoff from any treatment.
Compared to Affeldt et al. (2004), yields were higher in MC and lower in K, while both treatment had similar yields to those measured by Berkevich (2008). Corn forage quality was lower than the silage hybrids tested by Cox and Cherney (2011). With and without kura clover living mulch, rye yields were in the range found by Kazula (in review) but forage quality was lower than would be expected for boot stage harvest in southern Wisconsin (Kazula, in review; Stute et al., 2007) but this was almost certainly because harvested rye a little later than the boot stage. Despite the lower yields for individual crops in kura clover living mulch, the addition of an unfertilized rye forage crop to the kura clover living mulch system can make up for reduced corn yield (Table 4).
Total macro-aggregate stability was similar to values measured in other silt loams (Idowu et al., 2009; Jokela et al., 2009). The treatment effects measured during this study generally agree with those of Dell et al. (2008), who found no effect of rye following no-till corn silage on wet aggregate stability or soil organic matter, and Jokela et al. (2009), who reported similar, small, not always significant, increases in large macro-aggregate stability when kura clover living mulch was used with corn but no improvement from winter rye. We saw an increase in soil organic matter from kura clover living mulch, unlike Jokela et al. (2009) who saw no increase relative to monocrop cornon a higher organic matter soil.
While the differences in stable, large macro-aggregate and organic matter content were fairly small, they could still be increasing infiltration and saturated hydraulic conductivity and slowing surface sealing, especially during intense storms (Lado et al., 2004a and 2004b; Ramos et al., 2003; Vermang et al., 2007). The reduction of soil surface sealing is particularly important in the loess soils of southwestern Wisconsin, where high silt content and slopes create conditions conducive to runoff and erosion (Bradford et al., 1987; Ramos et al., 2003; Römkens et al., 1995). Furthermore, the surface soil depth analyzed (0 to 5 cm) could be obscuring even larger differences at the immediate soil surface. It should also be noted that these treatment differences developed over only 2 years and may continue to increase over additional years.
Because MC tended to have higher soil moisture than other treatments, it would likely be less resistant to penetration solely due to this difference (Vaz et al., 2001). While we observed lower penetration resistance in MC relative to other treatments below 7.5 cm, it may have been partly the result of the differences in moisture. Conversely, higher penetration resistance at shallow depths (0 to 5 cm) in monocrop corn relative to cover crop treatments would be even larger at equal soil moisture levels. Surface sealing in monocrop corn could be responsible for the higher penetration resistance (Ramos et al., 2003; Vermang et al., 2007). We did not observe differences from wheel traffic in penetration resistance but there were only three planting operations during the experiment, and none when the soil was wet, and their effect may have been minimal or obscured by previous management.
Simulated rainfall events were used to analyze relative treatment differences in quantity and content of surface runoff. Rainfall simulation, rather than natural runoff, was used to assure a sufficient number of runoff events during the course of the experiment and to represent seasonal conditions and crop growth stages throughout the year. The results of these simulations should be considered as relative treatment differences applicable to but absolute concentrations and losses may be different than during natural runoff events (Bormann et al., 2010). Natural rainstorms of the same intensity that we used during simulations would likely result in higher amounts of runoff with higher dissolved reactive P concentrations (Bormann et al., 2010). Because storms of the intensity which we simulated are relatively uncommon, treatment differences in time to runoff would likely lead to even larger treatment differences in runoff amounts and losses than we measured during less intense or less prolonged natural rainstorms. Mean time to runoff in monocrop corn was never more than 8 min while in other treatments it was often more than 15 min. A rainstorm with high intensity for only 15 min might not produce any runoff from one of the cover crop treatments while almost certainly resulting in runoff from monocrop corn.
While there were some differences in soil moisture, NH4-N, and NO3-N levels in surface soils (0 to 5 cm), these differences were either quite small, in the case of soil moisture, or did not correlate with the nutrient losses and concentrations measured in the runoff. Rather it appears that more ground cover was associated with lower amounts of runoff (Figure 1), suspended sediment (Figure 2), and soil loss (Figure 3). This was especially the case during the April, June, and September 2010 and May 2011 simulations. In October 2010, similar to the observations made by Laloy and Bielders (2010), soil surface disturbance and slope perpendicular ridging was fairly pronounced from no-till rye planting, and extensive surface pooling behind ridges was observed during simulations. Surface pooling may have been a more dominant factor leading to very low amounts of runoff from treatments with rye. Kura clover living mulch had a similar level of ground cover to the R and KR treatments but lost more water in runoff. Throughout the course of the experiment, the plant roots were probably responsible for increased organic matter in the soil, facilitating greater aggregate stability and infiltration and reduced runoff (De Baets et al., 2011; Kaspar et al., 2001; Lado et al., 2004b; Ramos et al., 2003; Vermang et al., 2007).
The October 2010 and May 2011 simulations showed that both kura clover living mulch and winter rye were effective at reducing runoff. While winter rye was more effective overall than kura clover in October 2010, their combined effect resulted in by far the lowest amount of runoff suggesting that environmental benefits of the two crops may have some compound effects. That said, the combined treatment (KR) did not perform nearly as well in May 2011, when both cover crops on their own (K and R) produced less runoff and total N losses. Soil disturbance associated with the winter rye planting would have smaller effects on the runoff in the spring, but this does not explain why the solo rye (R) was more effective than the combination (KR). While winter rye may produce substantial environmental benefits compared to kura clover during the fall, it may be due as much to the act of the planter on the soil as the actual presence of rye itself. The reductions in runoff from the kura clover and rye combination (KR) in October 2010 might also have been achieved through a very light tillage rather than planting operation or through strip tillage for corn planting (Bard, 2009, Kaspar et al., 2001). If this were the case, it might make more sense for some farmers using kura clover living mulch as they could simplify spring management.
Neither winter rye or kura clover living mulch performed as well as the alfalfa (Medicago sativa L.) and smooth bromegrass (Bromus inermis Leyss.) subjected to simulated rainstorms of the same intensity near the site of this research, although on somewhat less sloped plots than we used (Zemenchik et al., 1996 and 2002). Except for K in June and KR in October 2010, all of the treatments resulted in more soil loss and during all of the simulations, all of the treatments produced more dissolved reactive P losses than either alfalfa, smooth bromegrass, or their combination (Zemenchik et al., 2002). Gallagher et al. (1996) performed rainfall simulations on corn following spring-killed alfalfa and found soil loss in the same range as we found in cover crop treatments, although again, the average slopes were lower than those in our plots. The results of Zemechick et al. (1996 and 2002) and Gallagher et al. (1996) suggest that the cover crops we studied would likely perform similarly to alfalfa and alfalfa-grass mixtures in fields with equal slopes and probably better than corn following alfalfa. Eleki (2003) measured higher amounts of suspended sediment concentration in runoff from corn grown in kura clover but lower amounts of soil loss, runoff, and total P loss during much less intense natural rainstorms than were observed in this study. While kura clover living mulch and winter rye were quite successful in this study at reducing the environmental impacts from corn silage, the heavy kura clover suppression during spring 2010 likely limited the effects of the living mulch to the low end of what would be expected under typical management (Affeldt et al., 2004).
- Figure 3. Mean soil loss as a function of mean ground cover during rainfall simulations in 2010 and 2011.
- Table 4. Mean forage yield and quality of corn and rye forage crops.
- Table 6. Mean penetration resistance in monocrop corn and corn grown with kura clover living mulch, winter rye, and their combination.
- Table 7. Mean volumetric soil moisture measured in the spring of 2011 at the same time as penetration resistance.
- Table 8. Mean ground cover, soil moisture, and soil nutrient content at the site of rainfall simulations in monocrop corn silage and in corn silage grown with kura clover living mulch, winter rye, and their combination.
- Table 9. Mean runoff mass and content from monocrop corn silage and corn silage grown with kura clover living mulch, winter rye, and their combination.
- Figure 1. Mean water runoff as a function of mean ground cover during rainfall simulations in 2010 and 2011.
- Figure 2. Mean suspended sediment concentration as a function of mean ground cover during rainfall simulations in 2010 and 2011.
- Table 5. Mean soil aggregate stability and organic matter in monocrop corn silage and in corn silage grown with kura clover living mulch, winter rye, and their combination.
The environmental benefits realizable through the use of cover crops in corn production are becoming more and more necessary. Agricultural production on the vulnerable loess soils of southwestern Wisconsin and neighboring states is moving towards greater intensification and increased corn production (Juckem et al., 2008; Turnquist et al., 2004). At the same time, climate change has resulted in higher atmospheric moisture content and precipitation globally and storm intensity and size have increased throughout the northern corn belt and in southwestern Wisconsin (Huntington, 2010; Palecki et al., 2005; Todd et al., 2006). Corn silage or whole-plant bioenergy crops grown in this region will lead to soil degradation, decreased production over time, and environmental pollution unless steps are taken to protect and enrich the soils (Blanco-Canqui and Lal, 2009; Fageria et al., 2005; Grande et al., 2005a; Grande et al., 2005b; Lal, 2009; Weinhold and Gilley, 2010). Kura clover living mulch and winter rye cover cropping, by increasing ground cover and infiltration and reducing erosion and nutrient runoff, are valuable tools for farmers developing and sustaining agricultural production and environmental health in southwestern Wisconsin.
Areas needing additional study
While the environmental benefits of kura clover living mulch and winter rye cover cropping in corn silage production are substantial, forage production in kura clover living mulch needs on-farm testing to show that it is reliably productive and versatile at a field scale in order to appeal to farmers.
Short-term economic analysis comparing kura clover living mulch to conventional forage production in terms of management time, fertilizer costs, and forage yields is also needed to complement the analysis of cropping system effects on erosion and environmental pollution which mainly affect farm, and societal, economics over longer time scales.