Farmers could enhance crop diversity in their farming systems by interseeding cover crops in corn in late May and June in corn rotations in the Upper Midwest. Recommendations must be developed for cover crop species, seeding rates, and interseeding timings that optimize cover crop growth and enhance corn production. Weeds must be controlled, and cover crops must establish in this system. Cover crops influence soil health in long term studies; however, the influence of interseeded cover crops on soil enzymes, soil structure, and nutrient cycling has not been reported. In Michigan, two experiments were conducted from 2015-2017 and one experiment from 2017-2019. In the first experiment, annual ryegrass, crimson clover, oilseed radish and a mixture of the three species were broadcast interseeded at each of the V1 through V7 corn stages at a single seeding rate. Cover crop and weed density and biomass were measured during the growing season, at the time of corn harvest, and the following spring. Soil samples were taken in the spring in the year following interseeding and analyzed for inorganic N, extracellular enzyme activity, and aggregate stability. Corn was planted as an indicator crop and sampled for C and N content. In the second experiment, preemergence (PRE) and postemergence (POST) herbicides were applied, and cover crops interseeded at the V3 and V6 corn stages. Cover crops were evaluated in October for injury and stand loss. A greenhouse trial was also included to evaluate cover crop response to herbicides. In the third experiment, the same three cover crop species and a mixture of annual ryegrass and crimson clover were interseeded at three seeding rates in V3 and V6 corn. Establishment, biomass, and corn grain yield were collected using the same methods as previously described. Eight on-farm locations were interseeded with the same cover crop species at the 1X rate at the V3 and V6 corn stages. All plots were flown with a fixed-wing aircraft to measure canopy temperature. Small-plots were flown with UAV to acquire multispectral imagery to determine NDVI and NDRE. In years with normal or below normal precipitation, annual ryegrass and oilseed radish produced the highest biomass. Establishment improved when seeding on tilled soil compared with no-till soil. All cover crop species established, regardless of tillage, with above normal rainfall. Both annual ryegrass and crimson clover established when interseeded as a mixture at the seeding rates used. Increasing seeding rates usually increased biomass production. Cover crops could be interseeded at any time from V1-V7 corn if weeds were controlled. No cover crop species was competitive with summer annual weeds; annual ryegrass was the only species that overwintered and suppressed winter annual weeds. There were PRE and POST options for weed control with all cover crop species, but farmers must be mindful of herbicide and cover crop combinations. Delaying interseeding until V6 may reduce injury from some PRE herbicides. In the year of interseeding, cover crops did not reduce corn grain yield; therefore, remote imagery was not able to detect changes in corn health. Remote imagery detected cover crop establishment in the V3 interseedings prior to corn canopy closure; remote imagery did not detect less thermal stress where cover crops were interseeded. Annual ryegrass plots had reduced spring inorganic N content, and this sometimes translated to reduced N in the indicator corn crop. Success of broadcast interseeded cover crops is highly depended on adequate precipitation; this practice would be especially successful where summer rainfall is consistent or in irrigated systems. Benefits of cover crops are likely to be realized over multiple years of interseeding; farmers must balance goals of cover cropping with costs of seeding when selecting species, seeding rates, and weed control options.
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