Water use efficiency is not constant when crop water supply is adequate or fixed: The role of agronomic management

Increases in crop production per unit of water used is imperative for supplying adequate food, feed, and fiber in an environment where future water supplies are expected to decrease. Previous work on crop productivity per unit of water used (water use efficiency; WUE) has primarily dealt with crops grown under water limited conditions and have usually not considered crop management factors other than irrigation. Crop management can strongly influence yields when water is not limited. The aim of this paper is to demonstrate that transpiration per unit of productivity can vary greatly with agronomic management of crops when soil water supply is adequate or fixed. Moreover, when yield from crops with common development patterns are increased by better crop management and improved cultivars, WUE is also increased. In recent decades high yields of maize have been accomplished with increased fertilizer and cultivars that tolerate high plant populations and uniform spacing.Although transpiration and soil evaporation (ET) occur simultaneously in the field, they are difficult to measure as separate components. However, the crop simulation model CERES maize can reasonably estimate each component. The CERES Maize model was used to assess how plant population, genetic type and weather influence yields and WUE. Simulated yield response of an old and modern hybrid to a wide range of plant densities and uniformity patterns agreed reasonably well with observations suggesting that plant densities need to be near 10–11 plant m−2 and uniformly spaced to obtain near maximum yield and WUE for Midwest USA climate.