Project overview
Cover crop “cocktails,” or mixtures containing three or more species, are gaining popularity in organic vegetable production due to their potential to increase ecosystem services and enhance crop production. Research suggests that these mixtures may be more beneficial than single species cover crops, enhancing weed suppression, providing plant-available nitrogen, and enhancing soil biological activity.
However, further research is needed to understand individual species’ performance within mixtures and their ability to deliver multifunctionality. The dynamics of crop competition can impact certain species within a cover crop mixture, thus an understanding of each species’ growth characteristics and competitive interactions is essential.
This study aims to address the research gap in understanding the multifunctional impact of summer cover crop mixtures on organic vegetable production in the Upper Midwest. The study utilized summer cover crop species known to perform well in Upper Midwest conditions.
Farmer takeaways
- Cover crop biomass productivity did not increase with the number of species in the mixtures, and weed suppression from mixtures was similar to, but not superior to, the most effective monocultures.
- In a mixture, browntop millet can increase overall biomass production and provide weed suppression.
- Cowpea was the least productive and the least weed suppressive species tested, but it produced high-quality tissue with superior nutrient concentrations among monocultures and helped increase the quality of mixtures.
- Cover crops with a C:N ratio ≤40:1 may result in greater N mineralization, compared to those with a high C:N ratio (>50:1).
Project objectives and approach
Compare the performance of four warm-season cover crop species alone and in mixtures
The research took place in 2022 and 2023 at the Iowa State University Horticulture Research Station in Ames, IA, USA, on certified organic land.
Cover crop functional groups were represented by grasses, legumes, and nonlegume broadleafs. The following cover crop monocultures were included in the treatments: browntop millet (grass), buckwheat (nonlegume broadleaf), cowpea (legume), and sunnhemp (legume). A three-way mixture of browntop millet, buckwheat, and cowpea (3-CP); a three-way mixture of browntop millet, buckwheat, and sunnhemp (3-SH); and a four-way mixture of all four species (4-W) were also tested.
Assess the ability of cover crop mixtures to suppress weeds, contribute N, increase crop yield, and enhance soil microbial habitat
Cabbage (Brassica oleracea var. Caraflex) was transplanted on 9 August 2022 and 15 August 2023 and harvested mid-October 2022 and 2023. Cabbage heads (24 per treatment/rep/year) were then graded as marketable and nonmarketable due to insect damage or small size.
Soil samples were collected three times each year, to a depth of 15 cm. Soil samples were analyzed for nutrients, and permanganate oxidizable carbon (POXC) and microbial biomass carbon (MBC) as indicators of habitat for microorganisms. Anion exchange membranes (AEMs) inserted in the soil were used to quantify the NO3-N released from cover crop and soil mineralization during cabbage production.
Key findings
Productivity was not reliably increased in mixtures
Browntop millet and buckwheat monocultures produced the greatest biomass, and cowpea and sunnhemp monocultures produced the lowest. While mixtures were as productive as browntop millet (the best monoculture) in the first year, they did not exceed this productivity. Cover crop biomass productivity did not increase with the number of species in the mixtures.
Weed suppression dependent on cover crop biomass and performance of individual species
Grasses and nonlegume broadleaves were more effective at suppressing weeds than legumes in monoculture. Weed suppression was found to be negatively correlated with cover crop biomass (R2 = 0.57). The most productive treatments, browntop millet in 2022 and buckwheat in 2023, were the most effective at suppressing weeds. Weed suppression from mixtures was similar to, but not superior to, the most effective monocultures.
Performance of browntop millet, buckwheat, cowpea, and sunnhemp in mixtures
- Browntop millet exerted strong dominance in mixtures. Many researchers have also found highly productive grass species to exert strong interspecific competition in mixtures, leading to legume suppression.
- Buckwheat was competitive against browntop millet, but did not suppress legumes.
- Cowpea was suppressed to a much greater degree than sunnhemp in mixtures. If used in a mixture with browntop millet or other grass species, cowpea seeding rates should be maintained near the full seeding rate, or above 50 kg ha−1.
- Sunnhemp was competitive with browntop millet, and its low C:N ratio and high biomass production supported N contributions and weed suppression in cover crop mixtures.
Cover crop C:N ratios and tissue nutrient concentrations
The grass monoculture, browntop millet, had the highest C:N ratio (49:1), and the legume monocultures, CP and SH, had the lowest (<20:1). The C:N ratio of mixtures were greater than monocultures, which can be partly explained by the high proportion (58% on average) of browntop millet.
Certain cover crops can scavenge nutrients better than cash crops, and when the residue is incorporated, supply a labile form of the nutrient for subsequent crop uptake (Fageria et al., 2005). In this study, cowpea produced high-quality tissue with superior micro- and macronutrient concentrations among monocultures and increasing mixture concentrations in 3-CP and 4-W.
Decreased N contribution and cabbage yield with high browntop millet biomass
There is evidence of N immobilization from browntop millet (BTM) in 2022 as NO3-N mineralization was lowest in BTM plots (C:N ratio of 49:1), and more than 50% of cabbage heads in BTM plots were classified as small. On the other hand, in 2022, buckwheat biomass had a C:N ratio of 36:1 and similar cabbage yield to legume monocultures.
Limited impact on microbial biomass and labile carbon from mixtures
Significantly higher MBC following cover crop mixtures than monocultures in 2022 (but not in 2023) gives partial evidence of an increase in microbial populations, and infers mixtures may offer a more beneficial habitat for soil microorganisms.
Resources
Carey, A. M., & Nair, A. (2025). Evaluating ecosystem services of summer cover crop mixtures for organic cabbage production. Renewable Agriculture and Food Systems, 40, e8.
Read MoreLocation
IowaCollaborators
Ajay Nair, Iowa State University
Region
North Central
Topic
Soil Health, Crop Nutrient Management, Weed Management, Cropping Systems
Category
Vegetables/Fruits
Year Published
2025
