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 effect 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.
2. Study the effect of mixture rate on winter annual and summer annual weed populations.
3. Quantify trends in soil inorganic N dynamics across mixture treatments and PM use.
4. Evaluate the effects of mixture rate and PM use on soil microbial biomass and community functional diversity.
5. Evaluate the effects of mixture rate and PM use on vegetable yield and fruit quality.

Accomplishments/Milestones

Data collection for the second year of the experiment (first year funded by SARE) was completed from September 2009 – August 2010.

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 2009, in addition to a no cover crop control. Cover crops were killed and incorporated in mid-May 2010, and bell pepper and cucumber were grown with and without PM during the summer.

The experiment will be repeated again in 2010/2011, followed by final data analysis and dissemination of results. Preliminary results to date are discussed below.

As expected, 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. In 2010, spring densities across mixture treatments varied from 0 to 137 plants/m2 for vetch and 0 to 268 plants/m2 for rye. Data on the effects of mixture rate on relative establishment and winter survival for each species are currently being analyzed.

Relative biomass yields of vetch across mixtures varied greatly between the first and second years of the study, with monoculture dry yields of 950 lb/A in 2009 after a late 29 September planting date, and 5030 lb/A in 2010 after planting on 1 September. Rye biomass production was less sensitive to planting date, yielding 3320 lb/A and 3700 lb/A in monoculture in 2009 and 2010, respectively.

Total cover crop biomass yields across mixture proportions followed a gradient generally intermediate to the vetch and rye monoculture yields in 2010. 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).

All cover crop treatments significantly suppressed winter weed populations, with average density reductions ranging from 63 to 84 percent, and reductions in average weed biomass per plant from 65 to 76 percent, compared to the no cover crop control. However, no significant cover crop effect was observed 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. Cover crop mixtures with higher proportions of vetch generally resulted in higher soil nitrate concentrations over the course of the summer. However, both the magnitude and duration of the increases in soil nitrate following cover crop incorporation were overall greater under PM than without. This is likely due to a combination of higher mineralization rates and lower rates of nitrate leaching under PM. Our results suggest that PM can be an important tool for maximizing fertility benefits from incorporated cover crop residues.

Microbial communities from soils sampled 5 weeks following cover crop incorporation (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. The data are currently being analyzed; however, preliminary review has revealed lower estimates for microbial biomass under PM compared to bare soil at this sampling date.

In general, total yields of both bell pepper and cucumber were higher following cover crop mixtures with greater proportions of vetch. However, total yields were uniformly higher for peppers and cucumbers grown on PM, potentially due in part to the fertility benefits from PM discussed in Objective 5. Although increasing the proportion of vetch in the cover crop mixture generally benefitted 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

And through the following manuscript 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.

A field tour of the experiment has been scheduled for July 20, 2011 as part of the upcoming 2011 Ag Expo at Michigan State University, and additional presentations will be made at the 2011 American Society of Horticultural Science (ASHS) Annual Conference and the 2011 Great Lakes Expo. Additional scientific and grower-targeted articles will be published following completion of the field research.

Collaborators: