Precipitation and not cover crop composition influenced corn economic optimal N rate and yield

The effects of single species cover crops on corn (Zea mays L.) N requirement and grain yield are well studied throughout the U.S. Midwest. However, comparing cover crop mixes that include different compositions of grass and broadleaf species is limited. Fourteen corn N response experiments were conducted in South Dakota from 2018 to 2021. Fall cover crops planted after small grain harvest were mixtures of dominantly grasses, broadleaves, a 50/50 grass/broadleaf mixture, and a no cover crop control. Compared to the control, including a cover crop led to no differences in economic optimal N rate (EONR) and yield at zero N (0N) and yield at EONR 44%, 62%, and 83% of the time, respectively. As spring cover crop/residue biomass and its C and N content increased, corn yield at EONR decreased and EONR increased when including cover crops (R² = 0.36–0.56). Including cover crops reduced EONR and resulted in a similar yield when precipitation increased above 850 mm. When differences occurred with economic return from N, including a cover crop reduced economic return in 3 site-years (mean decrease of US$358 ha⁻¹) and in only 1 site-year did including a grass cover crop increase economic return from N (+US$335 ha⁻¹). Thus, in the first year of growing cover crops (i.e., grasses, broadleaves, or a grass/broadleaf mix) before corn, growers can normally expect some differences in EONR. However, with the appropriate rate of N, yield at EONR is maintained and any economic differences from N normally minimized.