A novel approach for optimizing the benefits of cereal-legume cover crop mixtures in vegetable cropping systems

A novel approach for optimizing the benefits of cereal-legume cover crop mixtures in vegetable cropping systems

Summary

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.

Objectives/Performance Targets

1. 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.

Accomplishments/Milestones

Field data collection for the first two seasons of this experiment funded by SARE (2009-10 and 2010-11) was completed in August of 2011.

A gradient of 7 rye:vetch cover crop mixture proportions from 100% rye (94 kg/ha) to 100% vetch (42 kg/ha) were sown on 1 September in both 2009 and 2010, in addition to a no cover crop control. Cover crops were killed and incorporated in mid-May of 2010 and 2011, and bell pepper and cucumber were grown with and without PM during the summers.

Final data analysis and dissemination of results are currently ongoing; however, preliminary results from the 2009-10 and 2010-11 seasons are discussed below.

In both years of the study, 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. Spring densities of rye and vetch across mixture treatments were similar in 2010 and 2011, and data on the effects of mixture rate on relative establishment and winter survival for each species are currently being analyzed, in preparation for publication.

In 2010, monoculture yields of vetch and rye shoots were 5030 and 3700 lb/A, respectively. In 2011, monoculture yields of vetch and rye were 4000 and 2950 lb/A, respectively. Cooler spring temperatures in 2011 likely contributed to the lower cover crop biomass observed that year.

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. The total amount of N in the cover crops generally increased with higher proportions of vetch in mixture, while the total residue C:N generally decreased.

The total density and biomass of winter weeds in each mixture treatment were sampled at the time of cover crop kill in the spring. The densities of summer weeds in bell pepper rows were sampled once during the growing season, as were the densities and biomass of seeded lambsquarters (Chenopodium album) and pigweed (Amaranthus powellii).

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 near 70 percent in 2011. The suppressiveness of the mixtures appears to be related to the relative species composition, with mixtures containing higher proportions of rye providing better winter annual weed control.

No significant cover crop effect was observed in either year on the density of summer weeds 5 weeks following cover crop incorporation, either for native populations in pepper rows or for seeded weeds. While the living cover crops are excellent weed competitors, the effectiveness of incorporated residues at suppressing weeds during the growing season (through allelopathy, for example) appears to be minimal.

Soil nitrate levels were monitored through biweekly soil sampling and analysis during the growing season. 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 less pronounced. 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, but that benefits may depend on the conditions during a given season. We are currently analyzing soil and weather data from 2010 and 2011 in an effort to explain the variation in our results.

Microbial communities from soils sampled 5 weeks following cover crop incorporation in 2010 (3 weeks following vegetable transplanting) were analyzed for microbial biomass using the chloroform fumigation-incubation (CFI) method, and for functional diversity through community-level physiological profiling (CLPP) using Biolog EcoPlatesTM. Microbial biomass was significantly lower under PM compared to the bare soil at this sampling date in 2010, but no significant differences were resolved among cover crop treatments. CLPP data from 2010 are currently being analyzed, and microbial biomass and CLPP analyses on 2011 soil samples are currently being conducted in the laboratory.

In both years, total yields of both bell pepper and cucumber were generally higher following cover crop mixtures with greater proportions of vetch. In 2010, total yields were uniformly higher for peppers and cucumbers grown on PM, potentially due in part to the higher inorganic N levels under plastic that year. In 2011, yields of pepper and cucumber grown without PM were higher than in 2010, and as a result, the difference between yields of vegetables grown with PM and without PM were generally reduced (for pepper) or negligible (for cucumber). These results are likely related to the N fertility observations from Obj. 3, and additional research is needed to determine under what conditions the use of PM is likely to provide a yield advantage for vegetables following cover crop incorporation. Furthermore, although increasing the proportion of vetch in the cover crop mixture generally benefited vegetable yields, evaluations of overall mixture performance must ultimately be based on a systems-level consideration of the numerous services that cover crops can provide, as well as their relative costs—a synthesis that will be the subject of future analysis.

Impacts and Contributions/Outcomes

To date, results from this research have been disseminated to the scientific and grower communities through presentations at the following conferences:

– 2010 International Horticulture Congress, Lisbon, Portugal
– 2010 Great Lakes Fruit, Vegetable, and Farm Market Expo, Grand Rapids, MI
– 2011 Weed Science Society of America (WSSA) Annual Meeting, Portland, OR
– 2011 American Society of Horticultural Science (ASHS) Annual Meeting, Waikoloa, HI
– 2011 Great Lakes Fruit, Vegetable, and Farm Market Expo, Grand Rapids, MI
– 2012 Michigan State University Organic Reporting Session, East Lansing, MI

And through the following manuscripts submitted for publication:

Hayden, Z.D., Ngouajio, M., and Brainard, D.C. 2010. Investigating component species proportions in a cereal-legume cover crop mixture under organic management. Acta Horticulturae. In review.

Hayden, Z.D., Brainard, D.C., Henshaw, B., and Ngouajio, M. 2012. Winter annual weed suppression in rye-vetch cover crop mixtures. Weed Technology. In review.

A field tour of the experiment was also given July 20, 2011 as part of the 2011 Ag Expo at Michigan State University. 40 farmers, gardeners, and researchers were in attendance.

Collaborators: