Cover crop mixtures did not produce significantly more cover crop biomass than monocultures

Browntop millet and buckwheat monocultures produced the greatest cover crop 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 ever exceed this productivity. Cover crop biomass productivity did not increase with the number of species in the mixtures.

Weed suppression is dependent on cover crop biomass production and performance, with cover crop mixtures performing similarly to the most effective monocultures

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; in other words, greater cover crop biomass elicited fewer weeds. Accordingly, the most productive cover crop treatments (browntop millet in 2022 and buckwheat in 2023) were the most effective at suppressing weeds. Weed suppression from cover crop mixtures was similar to, but not superior to, the most effective monocultures. 

Individual cover crop performance within mixtures varies by species and functional group, with grasses tending to dominate/suppress legume performance

• Browntop millet (grass) exerted strong dominance in the cover crop mixtures. Several other studies have found highly productive grass species to exert strong interspecific competition in mixtures, resulting in legume suppression. 
• Buckwheat (nonlegume broadleaf) was competitive against browntop millet, but did not suppress legumes. 
• Cowpea (legume) was suppressed to a much greater degree than sunnhemp in mixtures. If incorporated into 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 (legume) 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. 

C:N ratios of cover crop mixtures tended to be higher than those of the monocultures due to high proportions of the dominant grass cover crop species

The grass monoculture, browntop millet, had the highest C:N ratio (49:1), and the legume monocultures, cowpea and sunnhemp, 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%) of browntop millet in the mixtures.

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. 

Incorporating cover crop residues with high C:N ratios, such as browntop millet, may immobilize N and result in reduced cabbage yields

There was 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. 

• *N immobilization may occur when residues with a high C:N ratio are incorporated into the soil, due to the consumption of all/most plant-available N by microorganisms during residue decomposition.

Buckwheat, which had a slightly lower C:N ratio of 36:1, did not appear to result in N immobilization, as cabbage head sizes and yields were similar to yields from legume monocultures. 

Cover crop mixtures may enhance microbial populations slightly, but additional research is needed

Significantly higher microbial biomass carbon (MBC) following cover crop mixtures than monocultures in 2022 (but not in 2023) provides partial evidence of an increase in microbial populations, and infers mixtures may offer a more beneficial habitat for soil microorganisms.