Cover crops decrease maize yield variability in sloping landscapes through increased water during reproductive stages

Rolling hill style topography is a common feature of agricultural land throughout the United States. Topographic complexity causes subfield variation in soil resources such as water and nutrients, leading to a mosaic of high- and low-productivity zones that can shift from one year to the next due to weather. Stabilizing yields across these productivity zones using agroecological methods may improve land use efficiency, prevent unnecessary cropland expansion, and reduce the environmental impact of these systems. Here, we hypothesized that cover crops may help to reduce soil water and nutrient losses and increase the stability of subsequent maize yields across time and space. We performed a field study to evaluate the effect of a cereal rye (Secale Cereale L.) cover crop on maize (Zea mays L.) yield at three landscape positions (summit, backslope, and toeslope) in Central KY in 2018–2019, and calibrated the DSSAT v4.7.0.001 computer simulation program to test our hypothesis across a thirty-year period. Our field trial showed pronounced variability in maize yield across different landscape positions, ranging from 6.3 Mg ha⁻¹ in the backslope, to 12.2 Mg ha⁻¹ in the toeslope. Model simulations were consistent with results from our field trial and indicated that low yields in the backslope were primarily due to water stress, with >10 % yield reductions in 17 out of 30 simulated years relative to simulations under irrigated conditions where water was not limiting. In contrast, the toeslope and summit positions experienced >10 % yield reductions due to water stress in only 6 of the 30 years. Growing a cereal rye cover crop before maize reduced the frequency of water stress and raised maize yields in the backslope by 6% (500 kg ha^-1) on average, and 24 % (1235 kg ha^-1) during dry years. The coefficient of variation across all weather conditions and landscape positions was reduced from 33 % to 26 % when maize followed a rye cover crop compared to fallow. The yield benefits of the cover crop were associated with decreased soil evaporation and runoff that increased water availability during anthesis and late maize reproductive phases. Crop model simulations allowed us to evaluate and parse out the fundamental drivers of the interaction between cover crops and complex topography under different weather scenarios. Overall, our study demonstrates the outsized potential of cover crops to increase and stabilize grain yields in rolling hill landscapes and emphasizes the value of cover crops as a tool for ecological intensification.