Tradeoffs when interseeding cover crops into corn across the Chesapeake Bay watershed

CONTEXT: In annual cropping systems at high latitudes, an early onset of winter limits the establishment of cover crops in fall and ultimately the ecosystem services they provide such as nitrogen (N) leaching reduction. Interseeding cover crops into a standing cash crop can improve establishment before winter, but its practice is limited in both scope and scale. OBJECTIVE: To identify tradeoffs and establish regional patterns of interseeding success in the environmentally sensitive Chesapeake Bay Watershed of North America, we used the agroecosystem model Cycles to compare in silico systems in which cover crops were either interseeded into a standing crop of corn (Zea mays L.) or conventionally seeded after corn harvest. METHODS: In each of the 153 counties contained in the Chesapeake Bay Watershed, a representative field was simulated and either seeded with ryegrass (Lolium perenne L.), a blend of legumes, or a mix of legumes and ryegrass in a continuous corn sequence. The simulated corn yield, cover crop biomass production, N leaching reduction, and total N losses were compared between the cover crop scenarios and a monoculture corn control. RESULTS AND CONCLUSIONS: Simulations showed that conventionally seeded cover crops did not affect corn yields of the following year. However, when interseeded, the drag on corn yield in the same year averaged 6% per Mg of fall cover crop biomass. The corn yield reduction notwithstanding, interseeding reduced N leaching by 10% for legumes, 25% for the mix, and 50% for solo ryegrass compared to the no cover-crop control. There are other tradeoffs, such as an offset of N fertilizer inputs from N2 fixation when using legumes despite the less effective nutrient capture. Regardless of the cover crop type, interseeding up to latitude 41 degrees N tends to be competitive with the cash crop, but north of that threshold (or west into higher elevations) the mitigation of N leaching when interseeding seems to outweigh agronomic costs. SIGNIFICANCE: This study provides useful insight into watershed-scale agronomic and environmental processes without the constraints of establishing large-scale networks of field trials. Given these analyses, future field research in areas suitable for interseeding can focus on establishing legume cover crops in high-yielding fields with low residual mineral N, grass cover crops in manured or low-yielding fields with residual mineral N, and a mixture of both when N supply is uncertain and preventive management is needed.