CROPPING SYSTEMS FOR STRETCHING LIMITED IRRIGATION SUPPLIES

Declining groundwater levels in irrigated areas of the Great Plains ma y require a shift to limited irrigation or a return to dryland product ion. A field study was developed to determine the yields that could be attained in minimum till cropping systems that included winter wheat (Triticum aestivum L.), corn (Zea mays L.), soybean [Glycine max (L.) Merr.] and grain sorghum [Sorghum bicolor (L.) Moench] under dryland f arming, and limited and full irrigation. The limited irrigation alloca tion was 6 in./crop/year. The experiment began in 1981 on a Cozad silt loam (fine-silty, mixed, mesic Fluventic Haplustoll) at North Platte, NE, where average annual precipitation was 19.4 in. Management practi ces for fertilizer, herbicide, variety selection, and pest management were modified as improved varieties, technology, and products became a vailable. In the winter wheat (WW)-corn (C)-soybean (SB) rotation, 6-y r average yields for winter wheat were 33, 64, and 65 bu/acre for dryl and, limited, and full irrigation, respectively. Average corn yields w ere 90, 161, and 188 bu/acre for the three moisture regimes, whereas s oybean average yields were 29, 52, and 59 bu/acre. The limited irrigat ion yields were 99, 86, and 88% of fully irrigated yields for winter w heat, corn, and soybean. Marginal returns from limited irrigation were 10.9 bu/in. for corn, 4.3 bu/in. for soybean, and 5.9 bu/in. for wint er wheat. Poor stand establishment in heavy residue and chlorosis were problems in grain sorghum culture, which may limit its competitivenes s with corn in this area. This research demonstrated that minimum till age cropping systems and limited irrigation have possibilities for mai ntaining acceptable yield levels of corn, winter wheat, and soybean in areas of declining irrigation water.