Cover crops did not change optimal corn nitrogen rate over three variable precipitation seasons in the Western Corn Belt

Recommending the agronomic optimal nitrogen rate (AONR) for corn remains an elusive agronomic challenge in spite of many decades of research. Adding cover crops to corn production increases the challenge of determining optimum nitrogen (N) rates because of their impact on N cycling. We evaluated the AONR and the corn yield at the AONR following cereal rye, hairy vetch, a rye–vetch mixture, and a no-cover crop treatment in the cover crop-corn growing seasons of 2020–2021, 2021–2022, and 2022–2023 in an experiment that included six N rates (0, 45, 90, 180, 270, and 360 kg N ha⁻¹) in Eastern Nebraska. The only consistent pattern found across years was that cover crops did not decrease the corn grain yield at the AONR, nor did they increase the AONR compared to the no-cover crop treatment. In 2021, a year with near-normal precipitation, the corn yield at the AONR and the AONR were the highest of all 3 years following cover crop treatments. In 2022, the total rainfall during the corn-growing season was approximately half of the 30-year average, decreasing yield at the AONR across treatments. In 2023, growing season total precipitation was closer to normal (86% of the 30-year average), and although there was no significant effect of cover crop treatments on the AONR, there was a decrease in yield compared to near-normal precipitation in 2021. These results highlight weather as a dominant factor driving AONR and do not support the need for higher N rates following winter cover crops.