Improving Minnesota's Water Quality with Cash Cover Crops.

Many water bodies in Minnesota are impaired due to agricultural contaminants, with the major contributors being nitrate- and phosphate-laden fertilizers and manure. Agricultural fields are highly susceptible to nutrient loss from October through May when fields are fallow. New legislation may require farmers to plant buffers along the edges of waterbodies, and though the optimal width of these buffers is debatable, they would help reduce movement of phosphorous and some nitrogen into these waterbodies. However, nitrogen has a greater propensity to leach into groundwater, thus buffers would not fully address the water quality problem Minnesota is facing. Cover crops also can be used to retain these nutrients in agricultural fields. We evaluated how well four cover crops (winter rye, winter camelina, pennycress, and tillage radish) reduced nitrogen leaching and phosphorous movement in a relay crop production system with soybean. That is, cover crops were sown in autumn, and in spring soybean was planted into living camelina, living pennycress, herbicide-treated rye, and residue of winter-killed radish. Biomass, soil, soil water, and runoff water in cover crop treatments were analyzed for nitrogen and phosphorus. All four crops reduced nitrogen and phosphorous loss compared to the conventionally fallowed check treatment. However, radish and winter rye incorporated about 60% more nitrogen than winter camelina and about 160% more nitrogen than pennycress in the fall, which is a critical time when excess nitrogen and phosphorus in the field could contaminate water.  Seeds of both of these cover crops were harvested in late June, and the relayed soybean was harvested in September; thus, two crop harvests in one growing season. Consequently, a more practical plan for farmers to limit fertilizer movement may be to relay either winter camelina or pennycress with soybean, as these cover crops have the added benefit of providing a secondary cash crop.