Nitrogen fertilizer equivalence of red clover when inter-seeded into corn

Inter-seeding red clover (Trifolium pratense L.) provides an alternative method to incorporate cover crops into continuous corn (Zea mays L.) in the Upper US Midwest. Red clover is a leguminous cover crop that can grow in low-radiation environments and is winter hardy. Systems with red clover have demonstrated improved corn yield and a fertilizer N equivalence but understanding these effects with inter-seeding warrants further investigation. The objectives of this study were to determine the effect of inter-seeding red clover on (i) plant-available N during and after red clover decomposition, (ii) optimum N rates for corn, and (iii) corn yields. The experimental design was a randomized, complete block-split plot design, with cover crop as the main plot factor (treatments with and without inter-seeded red clover) and N-rate as a split-plot factor (N-rates between 0 and 315 kg-N ha⁻¹ in 45 kg-N ha⁻¹ intervals). Quadratic plateau response curves with a bootstrapping technique were used to determine differences in optimum N rate among treatments. Corn yields were evaluated with or without red clover inter-seeded following the first inter-seeding year. Red clover accumulated biomass values of 50 kg ha⁻¹ up to 300 kg ha⁻¹ pre-termination when inter-seeded with corn at the V4–V5 growth stage without detriment to yield. Corn yield was improved with clover treatments in one of four site years tested, resulting in a (15–17 kg ha⁻¹) lower N requirement that year. Overall, our results indicate that inter-seeding red clover into continuous corn did not provide an agronomically meaningful nitrogen fertilizer equivalence to the cropping system.