Assessing the Impact of Management Practices on on-Farm Soybean Water Productivity.

Although U.S. accounts for 38% of global soybean production, and produces 85 Mt annually, assessment of U.S. soybean on-farm water productivity (WP; kg seed per mm of water supply) has not been performed. Here we evaluated soybean WP using a database containing on-farm field yields, applied inputs, and management practices collected from 516 irrigated and rainfed fields during three years (2010–2012) in Nebraska (western U.S. Corn Belt). Water supply, including available soil water at sowing and in-season rainfall and irrigation, set an upper limit to productivity. The boundary function for the relationship between yield and water supply had a slope (= attainable WP) of 9.9 kg ha^-1 mm^-1  (3.7 bu ac^-1 in^-1) and x-intercept of 73 mm (3 in). A seasonal water supply of ca. 650 mm (25 in) appeared sufficient to maximize seed yield (i.e., yields near 6 Mg ha^-1 or 90 bu ac^-1) and an important number of irrigated fields exhibited water supply well above this value. Rainfed and irrigated fields were 31 and 20% below their respective attainable WP. Sowing date explained the largest portion of yield variation among fields, with yield penalty ranging from 32 to 52 kg ha^-1 d^-1 across regions (1/2 to ¾ bu ac^-1 d^-1). Yield penalty was observed in no-till fields, compared to reduced- or disked fields, in region-years with cooler early-season temperatures. Higher irrigated and rainfed yields were observed in fields that received starter fertilizer or P fertilizer. Analysis of this database indicated that (i) soybean producers obtained yields close (70-80%) to the attainable WP, and (ii) on-farm improvement of WP might be achieved by fine-tuning of current management practices, including earlier sowing date coupled with judiciously chosen tillage to achieve warmer soils in the springs, suitably applied nutrient fertilizer application, and better irrigation management.