Stability of Corn and Soybean Yield Ratios in Three Midwestern Environments

Abstract

Core Ideas Corn/soybean yield ratios estimate species × environment interactions.County yields from Kentucky, Iowa, and Nebraska (1972–2015) were used.Cropping systems had a greater effect on average ratios than productivity level.Year‐to‐year variation in the ratio decreased as the productivity level increased.Yield ratios increased significantly in only 37% of the counties (mostly in Iowa) Developing cropping systems to maintain productivity in future environments will require a better understanding of crop species × environment interactions. The ratios of county corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields (corn/soybean) from three midwestern states (Kentucky [31 counties], Iowa [47 counties] and Nebraska [irrigated, 33 counties]) (1972 through 2015) were used to evaluate this interaction. Yield estimates (converted to a weight basis) were obtained from the National Agriculture Statistics Service. Average yield ratios were similar in Iowa (2.96) and Nebraska (3.00) in spite of higher yields in Nebraska. The highest average ratio (3.39) occurred in Kentucky counties where soybean double cropped after winter wheat (Triticum aestivium L.) predominated. Yield ratios were remarkably stable (low CVs) over time in high‐yield environments, but stability decreased as county productivity (estimated by average corn yield) decreased (CVs increased from ∼8 to 23%). The mean yield ratios in Kentucky and Nebraska decreased significantly as the proportion of continuous corn in a county increased. The largest yield ratios in non‐double crop counties in Kentucky were associated with a larger proportion of continuous soybean in the rotation. Yield ratios increased significantly (p < 0.10) from 1972 to 2015 in only 37% of the counties (mostly in Iowa). The largest interactions between corn and soybean involved cropping systems and not fundamental physiological characteristics of these species. The consistency of mean yield ratios in rain fed and irrigated environments suggests that these crops may respond similarly to changes in precipitation.