RESPONSE OF SOYBEAN TO DEFICIT IRRIGATION IN THE SEMI-ARID ENVIRONMENT OF WEST-CENTRAL NEBRASKA

Abstract

Several factors, including multi-year drought, declining aquifer levels, and new water regulations, arecontributing to reduced availability of irrigation water in the semi-arid area of west-central Nebraska. Since many farmersin this area do not have enough water to meet the seasonal water requirements of crops like corn and soybean, maximizingyield produced per unit of water under deficit irrigation conditions is becoming increasingly important. This study wasconducted to quantify the grain yield response of soybean [Glycine max (L.) Merr.] to deficit irrigation, and to determinewhich seasonal water variables correlated best to soybean grain yield under deficit irrigation. The study was conductedduring 2002 at Curtis, and 2003 and 2004 at North Platte, Nebraska. Nine deficit irrigation treatments, including differentirrigation amounts and timings, were studied in 2002 and 2003, and eight treatments were studied in 2004. Soybean grainyields across years and sites were best related to the seasonal ratio of the actual crop evapotranspiration and the cropevapotranspiration when soil water was not limiting (ETd/ETw), and to the seasonal ratio of actual crop transpiration andcrop transpiration when soil water was not limiting (Td/Tw). Both of these seasonal ratios were linearly related to grain yieldwith R2 = 0.91 when combining data for all seasons. The crop water productivity (CWP) (yield per unit of seasonal ETd)linearly increased with both ETd/ETw (R2 = 0.72) and Td/Tw (R2 = 0.72), but was best correlated to the daily positive differencebetween the actual and the theoretical fraction of total available soil water in the root zone that can be depleted before cropwater stress occurred, accumulated for the entire season (seasonal pdiff) (R2 = 0.77). A linear relationship between thecumulative ETw and fraction of season (function of days after emergence) was found. This relationship developed for a givenlocation could be used to extrapolate seasonal ETw for in-season irrigation management. Poor correlation was found betweenCWP and other variables such as total irrigation, rain + irrigation, and total water. The results of this study can provide usefulinformation for soybean irrigators to make better management decisions under deficit irrigation conditions.