Spring Timing and Method of Cereal Rye Termination Influence Soil Nitrate.

Nitrate lost from agricultural fields is costly to farmers and diminishes water quality. Soil nitrate loss occurs when water movement and drainage from soils are high, most of which occurs from April through June in Iowa. Cereal rye, a cold-tolerant cover crop, can reduce nitrate loss by using nitrate when cash crops are not growing and using nitrate. Farmers can choose from a variety of herbicides and times at which to terminate cereal rye prior to corn or soybean planting; these management suites may affect nitrate retention. In a corn and soybean system in central Iowa, we investigated how cover crop presence, herbicide type, and herbicide timing affect soil nitrate, cover crop nitrogen accumulation, and cover crop decomposition. Herbicides included glyphosate and paraquat, which have different modes of action: inhibition of protein synthesis, and cell lysis, respectively. Herbicide timing included an early termination date two weeks before corn seeding, and a late termination date several days before soy seeding. Soil nitrate was sampled weekly throughout the season. From fall through spring, rye significantly reduced soil nitrate concentrations (p<0.05). In 2014 from mid-May to mid-June, treatments terminated with glyphosate had lower soil nitrate than treatments terminated with paraquat (p<0.05); this trend continued but was not significant in 2015. Compared to no cover crop and early-terminated cover crop treatments, soil nitrate concentrations in the late-terminated cover crop treatment were lower further into the summer (p<0.05). Preliminary results from a litter bag study on carbon and nitrogen decomposition from rye shoots suggest that the carbon-to-nitrogen ratio increased more rapidly in paraquat-treated plots, indicating faster nitrogen loss compared to termination with glyphosate. Results suggest that termination with glyphosate and late-termination will maximize nitrate reduction potential of cover crops when nitrate loss is greatest in Iowa, reducing agricultural impacts on water quality.