Oilseed Cover Crops Relay Planted with Soybean: Yields, Economics, and Nutrient Uptake across Minnesota.

Many agricultural watersheds in Minnesota have excessive phosphorus and nitrogen, much of which originate in agricultural fields that are fallowed from October through May. Autumn-sown winter cover crops can be used to retain these nutrients. Soil NO₃-N and PO₄^-3 -P levels and quantities of N and P sequestered by winter rye (Secale cereale), forage radish (Raphanus sativus), and the oilseed crops, winter camelina (Camelina sativa), and pennycress (Thlaspi arvense) were evaluated in a relayed cover crop/soybean production system at three sites spanning the north-south climatic gradient of Minnesota. Forage radish sequestered the most N in autumn, but winter-killed and had high soil NO₃-N levels in spring. Winter rye was terminated chemically by early May at each site, whereas the oilseed crops were allowed to grow into June to full maturity and their seeds were harvested. In autumn through early May, winter camelina and pennycress sequestered about 25% less N than winter rye. However, they often sequestered ≥ 2.5 times more N and ≥ 2 times more P than winter rye when compared at maximum seasonal biomass (up to 130 and 20 kg ha^-1, respectively), with some of this N and P coming from a spring application. Oilseed sequestration efficiency of applied N and P ranged from 44-120% and 23-40%, respectively. Winter camelina yields ranged from 600 to 1100 kg ha^-1, while pennycress yields ranged from 900 kg ha^-1 to 1550 kg ha^-1. When combined with yields of relay-cropped soybean, net income for relay-crop systems was generally equivalent to mono-cropped soybean.