Managing cover crop C:N ratio and subsurface-banded poultry litter rate for optimal corn yields

Cover crops can be used to provide some of the nitrogen (N) needs of a cash crop to complement mineral fertilizers or manure, but there has yet been limited work to describe corn (Zea mays L.) yield as a function of cover crop quality and N inputs. We investigated the response of corn yield to gradients of both preceding cover crop C:N ratio and poultry litter (PL) application rates in Beltsville, MD during 2012-2014. To achieve different C:N ratios of the cover crops, hairy vetch (Vicia villosa Roth. "Groff") and cereal rye (Secale cereale L. "Aroostook") were seeded in a replacement series of six seeding rate proportions, resulting in shoot C:N ratios of 9.2:1 to 152:1 across years. For each hairy vetch/cereal rye sown proportions, PL was side-dressed at corn V5-V8 stage in subsurface bands (SSB) at four targeted rates: Zero, P-based (67 kg plant available nitrogen [PAN] ha(-1)), N-based (135 kg PAN ha(-1)), and excess N and P (269 kg PAN ha(-1)). We found that corn yield followed a linear-plateau relationship across these two dimensions. Within the linear region, each unit increase in log-scaled cover crop C:N ratio resulted in a yield decrease of 2.56 & PLUSMN; 0.26 Mg ha(-1) at a given rate of SSB PL. To optimize corn yields, we describe a model where each unit increase in log-scaled cover crop C:N ratio required an additional 45.9 & PLUSMN; 6.22 kg PAN ha(-1) from SSB PL. Yields following winter fallow were typically intermediate to the range of yields observed following the gradient of cover crop C:N ratios. We did not find significant differences in corn yield responses when comparing SSB PL to at-planting incorporated or broadcast PL; we also found no significant differences between SSB PL and surface-banded urea ammonium nitrate. Taken together, our approach of modeling yield response across two dimensions can be widely used to guide adaptive N management in subsequent cash crops following winter cover crops, thereby balancing both economic and environmental objectives in cover crop-based cropping systems.