Interactive effect of planting date, cultivar maturity, and irrigation on soybean yield and water productivity

Soybean ( Glycine max [L.] Merr.), a predominantly rainfed crop in the upper US Mid‐South, often experiences yield reductions due to water stress, which has led to increased irrigation adoption. The optimum cultivar maturity group (MG) to maximize yield may differ between rainfed and irrigated conditions and depend on the planting date (PD). It is essential to account for the crop water use efficiency (WUE; yield produced per total growing season evapotranspiration) and total water productivity (WP; yield per total water supply from irrigation plus precipitation) to develop these recommendations. Field experiments were conducted from 2017 to 2020 in Lexington, KY, under irrigated and rainfed conditions, with May and June PDs, and 16 cultivars of MG 2–5. Results revealed variable responses to irrigation due to differences in precipitation across years. Three out of 4 years experienced water stress, with a yield gap ranging from 11% to 54% ( p < 0.001) depending on the year, PD, and MG. Yield responses among MGs differed by year, PD, and irrigation availability ( p < 0.001), yet MG 4 cultivars were still among the highest yielding across all scenarios. Irrigation increased WP and WUE in the driest year, but reduced WP in wetter years depending on the cultivar MG. These results demonstrate that the benefits of irrigation depend strongly on seasonal water availability, but that water may have little effect on optimum cultivar MG recommendations. The contrasting effect of irrigation on WP and WUE highlights the need to evaluate different metrics of water use efficiency in future studies.