Rusch, Hannah2019-12-112019-12-112019-08https://hdl.handle.net/11299/208955University of Minnesota M.S. thesis. August 2019. Major: Applied Plant Sciences. Advisors: Axel Garcia y Garcia, Jeffrey Coulter. 1 computer file (PDF); ix, 70 pages.The incorporation of cover crops into the maize (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation in the U.S. upper Midwest may improve economic and environmental sustainability. For example, cover crops grown during the fallow period between maize harvest and planting can reduce the loss of essential plant nutrients like nitrogen (N) to ground and surface waters thereby reducing surface and subsurface quality. However, long, cold winters in the upper Midwest region make selection of successful cover crop species and associated management practices a challenge. This study was conducted to evaluate the establishment and growth of a variety of cover crop monocultures and mixtures across multiple environments and their effect on maize growth and yield and N fate in the cropping system. Two experiments were conducted in Grand Rapids, Lamberton, and Waseca, MN from fall 2016 through spring 2019 to examine whether six cover crop strategies interseeded into maize at the four- to six-leaf collar stage (spring-interseeded) and at physiological maturity (fall-interseeded) compromised maize growth or yield. Annual ryegrass (Lolium multiflorum L., AR) and cereal rye (Secale cereale L., CR) were evaluated as monocultures and in mixtures with crimson clover (Trifolium incarnatum L., CC) and forage radish (Raphanus sativus L., FR). Differences in cover crop canopy cover and biomass were observed at Waseca in 2018. Greater accumulated growing degree days resulting from an earlier interseeding that year did not translate into increased cover crop canopy coverage or biomass of fall-interseeded cover crops compared with 2017. Differences in cover crop canopy cover and biomass among spring-interseeded cover crop strategies were observed at fall frost at all locations in 2017 and at Grand Rapids in 2018. Cover crop canopy cover and biomass at cover crop spring termination, soil moisture at maize planting, maize aboveground biomass and yield were unaffected by the regrowth of fall-interseeded cover crop strategies with CR. Similarly, maize aboveground biomass or yield were not affected by spring-interseeded cover crop strategies. These results highlight the potential for a variety of cover crop strategies to be interseeded into maize without negatively influencing maize production in the U.S. upper Midwest. Next steps might include exploring the influence of the cover crop strategies on soybean production to identify the suitability and optimal placement of cover crops within the maize-soybean rotation. High variability characterized the effect of cover crops interseeded into maize on N fate. Interseeded cover crops had no effect on soil NO3-N in a well-drained loam soil but were found to reduce soil NO3-N relative to no cover in both the 0-20 cm and 20-40 cm layers on moderately well drained and somewhat poorly drained clay loam soils. Fall-interseeded cover crops with CR reduced NO3-N in the soil solution at all three study locations. However, at Grand Rapids, differences in NO3-N concentrations may be due to porous soils and there appear to be thresholds of cover crop growth at Lamberton and Waseca below which cover crops do not reduce NO3-N concentrations in soil solution. Highly variable cover crop N accumulation results make it unclear which cover crop strategy poses the greatest potential for immobilizing N at each location. At Grand Rapids, greater N accumulation occurred in spring-interseeded cover crops than in fall-interseeded cover crops likely because more growing degree days were accumulated when cover crops were interseeded at four- to six-leaf collar stage maize. Cover crops with AR at Grand Rapids accumulated more N than those with CR when spring-interseeded, and the AR monoculture accumulated more N than mixtures with AR. This suggests that spring-interseeding of AR into maize may hold the most promise for Grand Rapids. At Lamberton and Waseca, spring- and fall-interseeded mixtures with AR + CC + FR and CR + CC + FR accumulated more N than both monocultures and mixtures of AR + CC and CR + CC. However, they did not always accumulate significantly more N than other treatments. Thus, the 3-species mixtures may be as effective as or better than other cover crop treatments for N scavenging at Lamberton and Waseca. Future work could examine increasing the seeding rate and using a drill to interseed cover crops at the four- to six-leaf collar stage to enhance the capacity of cover crops to provide N loss reduction services to the maize cropping system.enannual ryegrasscanopy covercereal ryecrimson cloverN accumulationyieldThesis or Dissertation