Final Report for GNC09-108
The unique and complementary traits of cereal rye (Secale cereale L.) and the legume hairy vetch (Vicia villosa Roth) make winter annual cover crop mixtures of the two species promising for vegetable cropping systems. Informed management of the relative species proportions in the mixture could provide an important means of optimizing performance to serve various farmer goals. A variation of the replacement series experimental design was used to investigate 1) how relative species composition (seeding rates) influence biomass characteristics of cover crop mixtures, and 2) the interactive effects of mixture residues and the use of black polyethylene mulch (PM) on cover crop mixture performance in bell pepper and slicing cucumber cropping systems, based on a systems-level investigation of agro-ecological services that cover crops can provide. Results show that cover crop mixture proportion and the use of PM affect weed communities, soil chemical and biological characteristics, and crop productivity.
Multi-species cover crop mixtures can provide a means of combining the strengths of different plant functional groups in a single field while also moderating their individual weaknesses. Cereal-legume cover crop mixtures are of particular interest because they provide the opportunity to effectively suppress weeds, control erosion, and scavenge leachable nitrate while also fixing atmospheric nitrogen.
Winter annual cover crop mixtures of cereal rye (Secale cereale L.) and the legume hairy vetch (Vicia villosa Roth) have shown promise in previous studies, and their winter hardiness makes them well suited for production in the North Central region. Alone, the notable ability of rye to suppress weeds, scavenge residual nitrate, and control erosion is often tempered by the high C:N ratio of its residues and the threat of subsequent yield losses due to nitrogen immobilization (Allison 1966, Wagger et al, 1998). Rye-vetch mixtures, however, have exhibited moderation of the total C:N ratio of residues without sacrificing benefits characteristic of rye, all while contributing significant amounts of fixed nitrogen to the system (Clark et al., 2007a, 2007b, 2007c; Ranells and Wagger, 1996, 1997a, 1997b; Teasdale and Abdul-Baki, 1998). In addition, total dry matter yields in rye-vetch mixtures can be greater than yields of either species in monoculture, and total N release from mixture residues can approach the amount released from vetch monocultures (Rannells and Wagger, 1996).
The performance of any 2-species cover crop mixture with respect to traits of interest (e.g. total biomass production and residue quality, weed suppression, nitrogen recycling and mineralization dynamics, and yields of subsequent crops) will theoretically vary along a continuum from 100% species A to 100% species B. For certain environmental conditions and farmer goals, an optimum mixture rate should exist somewhere between the two extremes, where the respective strengths of each species are balanced to provide maximum benefit. Few studies to date (Clark et al, 1994) have evaluated rye-vetch cover crops based on more than a single mixture proportion, and more thorough research on the relationship between mixture proportion and cover crop performance could ultimately lead to more-informed seeding rate recommendations that consider growing conditions, farmer goals, and crop management practices.
The replacement series is an experimental design where treatments consist of a pure stand of each component species and a gradient of species mixtures, allowing the researcher to observe how species proportions influence interspecific competition and mixture performance. The design has been applied in agricultural contexts most often to evaluate crop-weed interactions and the performance of cash crop bicultures, but with appropriate interpretation, the design is also suited for investigating cover crop mixtures (Jolliffee, 2000).
Furthermore, the use of black polyethylene mulch (PM) is an industry standard for large-scale commercial organic and conventional production of bell peppers and slicing cucumbers. By distinctly altering the soil microclimate through increasing soil temperatures and alteration of soil moisture dynamics, PM may have a substantial effect on the relative performance of cover crop mixtures, particularly with respect to nutrient mineralization and leaching (Clarkson et al, 1960; Tarara, 2000). Because many smaller scale organic growers reject the use of PM based on economic and environmental grounds related to its use and disposal, results for cover crop performance derived under both management practices can provide for broader applicability across the spectrum of vegetable producers.
The overall objective of this study was to improve our understanding of how species proportions (based on seeding rates) of a mixture of cereal rye and hairy vetch influence cover crop performance in a vegetable production system with respect to crop grown and plastic mulch use. Performance was evaluated based on a systems-level approach to data collection, encompassing the following specific objectives:
- Quantify trends in cover crop establishment and total residue quantity and quality across rye-vetch mixture rates.
Study the effect of mixture rate on winter annual and summer annual weed populations.
Quantify trends in soil inorganic N dynamics across mixture treatments and PM use.
Evaluate the effects of mixture rate and PM use on soil microbial biomass and community functional diversity.
Evaluate the effects of mixture rate and PM use on vegetable yield and fruit quality.
The study was conducted for two seasons, alternating between adjacent fields, from 2009 to 2011 on a loamy sand soil at Michigan State University’s Horticulture Teaching and Research Center (HTRC) in Holt, Michigan. The experiment was a split-split plot randomized complete block design with 4 replications. The whole plot factor was cover crop mixture treatment, with levels following a proportional replacement series design including the following proportions of rye:vetch (by seeding rates): 100:0, 83:17, 67:33, 50:50, 33:67, 17:83, 0:100 and 0:0 (a no cover crop control). The rye monocrop was seeded at 94 kg/ha and the vetch monocrop at 42 kg/ha.
The subplot factor was cash crop, with two levels: bell pepper (Capsicum annum) and slicing cucumber (Cucumis sativus). The sub-subplot factor was black polyethylene mulch (PM) use, with two levels: crop grown with or without PM. Following cover crop kill and incorporation in the spring, two rows of each crop were grown during the summer in each main plot, with one row grown on PM and one row grown on bare ground.
All aspects of field management were carried out in accordance with USDA organic guidelines. Bell pepper and slicing cucumber were chosen for inclusion in the study because of industry significance, their common culture on PM, and as model crops for evaluation of the cover crop mixtures due to their contrasting nutrient requirements and timing of peak nutrient demand.
The dates of key field operations and data collection are summarized in Table 1. Cover crop treatments were broadcast sown by hand into 6.7 x 8.5 m (22 x 28 ft) and 6.1 x 7.6 m (20 x 25 ft) whole plots in 2010 and 2011, respectively, and lightly incorporated to a depth of 5 cm. A grid system was used during seeding to help ensure uniformity (Figure 1).
Cover crop density by species and both above- and below-ground biomass were sampled prior to cover crop kill from 4 20x25 cm quadrats in each whole plot, and samples of shoot biomass were analyzed for total carbon and nitrogen. Cover crops in the field were subsequently flail mowed and incorporated into the soil using a rototiller.
At the time of cover crop biomass sampling, winter annual weed density and biomass were also sampled from the 4 quadrats in each whole plot. Following cover crop incorporation and cash crop establishment, total summer weed density and species composition was sampled in late June from 2 20x25 cm quadrats established in each non-plastic pepper row. In addition, a fixed number (by volume) of supplemental seeds of the summer annual weed species Chenopodium album (common lambsquarters) and Amaranthus powellii (powell amaranth) were sown into 70 cm rows in each whole plot at the time of crop transplanting. Germination rates were quantified by counting and removing germinated seedlings. Ten seedlings of each species were left in each row to grow for 4 weeks, followed by harvest and average biomass dry weight measurement.
Bell pepper (variety Paladin) and slicing cucumber (variety Cobra) transplants were grown in the greenhouse according to organic practices prior to field transplanting at the beginning of June. Bell peppers were grown in staggered double rows (18 inch plant spacing and 12 inch inter-row spacing) with each treatment (cover crop mixture x PM use) consisting of 20 data plants. Slicing cucumbers were grown in single rows (18 inch plant spacing) with each treatment containing 12 data plants. Both PM and no PM crop rows were drip irrigated and otherwise managed according to accepted commercial practices (Figure 2). Following vegetable harvests, fruit was graded according to market specifications and average yields were quantified in terms of fruit number and weight.
Composite soil samples were collected from each whole plot prior to cover crop seeding and following cover crop incorporation. In addition, soil samples were collected bi-weekly throughout the 2010 and 2011 growing seasons from each treatment (cover crop mixture x PM use) for bell pepper rows, omitting cucumber rows in the interest of time constraints and costs of analysis. All soil samples were analyzed for NO3- and NH4+ concentration by extraction with 1 M KCl and subsequent colorimetric analysis.
Subsamples of soil collected roughly 3 weeks following bed preparation and transplanting from PM and no PM treatments within pepper rows for select cover crop mixture treatments (Rye:Vetch 100:0, 50:50, 0:100, and 0:0) were used for microbial analyses. Soils were transported from the field and stored at 4C until analysis. After sieving, soil microbial biomass carbon was estimated in the lab following the chloroform fumigation-incubation (CFI) method as described in Horwath et al (1996), and functional diversity of microbial communities was evaluated through community-level physiological profiling (CLPP) using Biolog EcoPlatesTM (Garland, 1997; Stefanowicz, 2006). The Shannon-Wiener diversity index (H) was calculated as
H = - Sum[pi(ln(pi))]
where pi is the ratio of the substrate use of each plate well to the sum of the substrate uses of all wells (Zak et al., 1994).
Spring densities of rye and vetch across mixture treatments were similar in 2010 and 2011, and in both years, rye and vetch seeding rates were highly correlated with observed field densities, suggesting that the proportion of each species sown in mixture is a good predictor of species proportions in the resulting cover crop stands. Cover crop mixture rate did not have a significant effect on relative establishment or winter survival of hairy vetch in either year.
In 2010, monoculture yields of vetch and rye shoots were 4900 and 3700 lb/A, respectively, and in 2011, monoculture yields of vetch and rye were 2700 and 2950 lb/A, respectively. Cooler spring temperatures in 2011 likely contributed to the lower cover crop biomass observed that year, particularly for vetch.
In both years, yields of rye and vetch across the different mixture proportions followed a gradient generally intermediate to the vetch and rye monoculture yields. However, total residue quality varied significantly across mixture treatments. With higher proportions of vetch and lower proportions of rye in the mixture, the total amount of N in the cover crop residues generally increased, while the total residue C:N fell. All mixture treatments, with the exception of the rye monoculture, had a total residue C:N less than 30:1.
In both years, all cover crop treatments significantly suppressed winter annual weed populations. Rye monocultures generally provided the greatest level of suppression, reducing weed biomass by over 95 percent compared to the control in 2010 and 2011. Although the vetch monoculture reduced weed biomass by 90 percent in 2010, suppression by vetch was only near 70 percent in 2011, suggesting that suppression by vetch may be less robust in the face of year-to-year variability in environmental conditions than rye. The suppressiveness of rye-vetch mixtures appears to be driven by the relative species composition, with mixtures containing higher proportions of rye generally providing better winter annual weed control.
However, no significant cover crop effect was observed on the total density of native field populations of summer weeds sampled 5 weeks following cover crop incorporation. Similarly, the germination of sown seeds of Amaranthus powellii and Chenopodium album were not significantly affected by cover crop treatment. Cover crop influence on average biomass production of the two species was also not significant, though variability in the data may be masking real effects.
Our results demonstrate that the living cover crops are excellent weed suppressors, and mixtures with higher proportions of rye tended to provide greater control. Interestingly, these mixtures also exhibited lower light interception (less shading of the soil surface), suggesting that weed suppression from rye is likely derived in large part from allelopathic effects and/or the depletion of soil resources (water and nutrients). However, the practical effectiveness of incorporated residues at suppressing weeds during the growing season (through allelopathy or nitrogen effects on weed germination and/or growth, for example) appears to be minimal.
In both years, cover crop mixtures with higher proportions of vetch generally resulted in higher soil nitrate concentrations over the course of the summer. In 2010, both the magnitude and duration of the increases in soil nitrate following cover crop incorporation were overall greater under PM than without. However, in 2011, differences in soil nitrate concentrations between PM and no PM treatments were much less pronounced.
The effect of mixture treatment on soil nitrate concentrations generally reflects the differences in total N content of the cover crop residues. Furthermore, the rye monoculture was the only cover crop treatment with a total residue C:N greater than 30:1, suggesting that N immobilization may also be playing a role in the low soil nitrate concentrations observed following that treatment, particularly in 2010. Higher nitrate concentrations under PM would likely be due to a combination of higher mineralization rates and lower rates of nitrate leaching. Our results suggest that PM can be an important tool for maximizing fertility benefits from incorporated cover crop residues, particularly for high N, readily decomposable materials like hairy vetch. However, benefits may depend on conditions during a given summer, and differences in soil temperatures and the timing of leaching rains likely contributed to the differences observed between the 2010 and 2011 growing seasons.
Three weeks following plastic mulch application (and 5 weeks following cover crop incorporation), soil microbial biomass carbon was significantly lower for treatments with PM than for those without it, both in 2010 and 2011. However, no significant differences in microbial biomass were resolved among cover crop treatments in either year. Previous studies have demonstrated reductions in microbial biomass under black plastic mulch, which may result from a combination of higher temperature extremes, lower oxygen levels, and differences in the pattern of microbial community growth over time in soils that are covered with PM (Moreno and Moreno, 2008). Additional research is needed to better characterize the influence of PM and residue quality on soil microbial communities, and to better understand how these short term changes can be utilized to serve production goals.
The functional diversity of the soil microbial communities, as measured by the Shannon-Wiener diversity index, also varied among some treatments. However, the magnitudes of the differences were small, and significant interactions between PM use, cover crop treatments, and the two years of the study make interpreting the results difficult. Additional research will be necessary in order to understand the specific roles of plastic, residue quality, and time in affecting soil microbial functional diversity.
In 2010 and 2011, total marketable yields of both bell pepper and slicing cucumber were generally higher following cover crop mixtures that contained greater proportions of vetch. In 2010, yields were uniformly higher for peppers and cucumbers grown on PM, likely due in part to the higher inorganic N levels observed under plastic that year. In 2011, however, yields were overall higher than in 2010, and the difference between yields of vegetables grown with PM and without PM were generally reduced (for pepper) or negligible (for cucumber).
Over the course of the study, vegetable yields were closely correlated with soil inorganic N levels during the growing season. Higher N levels (and higher vegetable yields) were associated with 1) cover crop mixtures that contained higher proportions of vetch, and 2) the use of plastic mulch. In 2011, the year that yield advantages from PM were lowest, the differences in soil N levels under PM and without PM were also greatly reduced. These results highlight the importance of N dynamics in the effects of rye-vetch mixture proportions and PM use on vegetable yields, and additional research is needed to investigate how residue quality and plastic mulches can best be managed to optimize N fertility. Furthermore, the yield increases observed following high vetch cover crops and PM use need to be viewed in the context of the higher seed and supply costs that come with these management decisions. A partial budget analysis would be helpful in quantifying the relative economic benefits of any observed yield gains across the various cover crop mixtures and PM use.
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Clark, A. J., Meisinger, J. J., Decker, A. M., and Mulford, F. R. 2007a. Effects of a grass-selective herbicide in a vetch-rye cover crop system on corn grain yield and soil moisture. Agronomy Journal 99:43-48.
Clark, A. J., Meisinger, J. J., Decker, A. M., and Mulford, F. R. 2007b. Effects of a grass-selective herbicide in a vetch-rye cover crop system on nitrogen management. Agronomy Journal 99:36-42.
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Educational & Outreach Activities
Presentations at Scientific Meetings:
Hayden, Z.D., M. Ngouajio, and D.C. Brainard. 2010. Optimizing component species proportions in a cereal-legume cover crop mixture under organic management. 28th International Horticultural Congress, Lisbon, Portugal, 22-27 August. (Poster)
Brainard, D.C., B.E. Henshaw, Z.D. Hayden and M. Ngouajio. 2011. Tillage and cover crop effects on winter annual weeds in vegetable cropping systems. Abstract no. 141. Weed Science Society of America Annual Meeting, Portland, OR, February (Poster)
Hayden, Z.D., M. Ngouajio and D.C. Brainard. 2011. Rye-vetch cover crop species proportion and polyethylene mulch affect total biomass production, soil nitrate accumulation, and bell pepper yield. ASHS Annual Conference, Waikoloa, HI, September 25-28. HortScience 46(9): S258. (Poster)
Presentations at Extension Meetings:
Hayden Z.D., M. Ngouajio, and D.C. Brainard. 2011. Mixing cereal rye and hairy vetch for improved performance: do mixture proportions matter? Great Lakes Fruit, Vegetable, and Farm Market Expo. Grand Rapids, MI. December 6-8. (Oral)
Hayden, Z.D., M. Ngouajio and D.C. Brainard. 2012. Rye-vetch proportion and plastic mulch affect cover crop biomass production, soil nitrate, and bell pepper yield. MSU Organic Reporting Session, Kellogg Center, East Lansing, MI, March 2. (Poster —1st Place Poster Competition).
Hayden, Z.D. 2011. Research update. Michigan Ag Expo Organic Tour, Michigan State University, East Lansing, MI, 20 July.
Hayden, Z.D., M. Ngouajio, and D.C. Brainard. 2012. Investigating component species proportions in a cereal-legume cover crop mixture under organic management. Acta Hortic. 933: 363-369.
Hayden, Z.D., D.C. Brainard, B. Henshaw, and M. Ngouajio. 2012. Winter annual weed suppression in rye-vetch cover crop mixtures. Weed Technology (in press).
Hayden, Z.D. 2013. Optimizing cereal-legume cover crop mixtures in vegetable cropping systems through replacement series analysis and investigation of staggered seeding. Ph.D. Dissertation. Michigan State University. In progress.
This study has provided useful baseline information on 1) how relative species proportions (seeding rates) influence biomass characteristics of rye-vetch cover crop mixtures, and 2) the interactive effects of mixture residues and the use of black polyethylene mulch (PM) on cover crop mixture performance in bell pepper and slicing cucumber cropping systems.
- Seeding rates were shown to reliably predict rye-vetch cover crop stand composition and total residue quality (both total N content and C:N ratio).
While all cover crop treatments suppressed winter annual weeds compared to a control, mixtures with higher proportions of rye were most effective. However, cover crop residues did not have any significant suppressive effect on summer annual weeds capable of competing with vegetable crops.
N fertility is likely a key driver in the effects of cover crop mixture proportion and plastic mulch use on vegetable yields. Cover crop mixtures with greater proportions of vetch generally resulted in higher levels of soil N over the course of the summer and higher pepper and cucumber yields.
Similarly, where PM use resulted in higher levels of soil N, it also resulted in significant yield advantages. However, yield and N fertility benefits from PM varied between years of the study, suggesting that environmental conditions (such as temperature and rainfall) may be important determinants of outcomes from PM in any given season.
A greater understanding of how management decisions (such as mixture seeding rates and PM use) broadly affect services within a production system is a first step toward more-informed and adaptive seeding rate recommendations that can consider site-specific conditions, farmer goals, and crop management practices.
This project has promoted greater awareness among vegetable growers of the potential benefits and tradeoffs of rye-vetch cover crop mixtures, both with and without the subsequent use of black plastic mulch. In particular, farmers exposed to this research should have a better understanding of how adjusting the ratio of rye:vetch may affect cover crop performance with respect to various needs in their cropping system, allowing them to make more-informed decisions about mixture seeding rates. Over the course of the project period, nearly 150 individuals (mostly growers) were directly exposed to our research findings through extension events, including a field demonstration during the 2011 Michigan Ag Expo Organic Tour, and through an invited talk given during the cover crop session of the 2011 Great Lakes Fruit, Vegetable, and Farm Market Expo.
Areas needing additional study
In order to get the most from cereal-legume cover crop mixtures, there is still much to learn about the factors influencing competitive outcomes in mixture stands and about managing residue quality and plastic mulch for soil fertility. Some important questions to address through future research include:
- How do different soil types and baseline fertility levels affect biomass production in rye-vetch stands?
Does rye-vetch mixture proportion influence vetch nitrogen fixation?
Can other management approaches, such as staggered sowing of the component species, be used to moderate competition in rye-vetch mixtures and influence biomass production?
How can residue mixtures be manipulated to maximize the synchrony of N release with crop demand?
What is the relative nitrate leaching risk across rye-vetch mixture proportions, as well as the extent to which it is mitigated by PM use?
What environmental conditions are most important for determining the magnitude of the yield benefit provided by PM?
How do relative economic benefits compare across the various cover crop mixtures and PM use?