Cropping Systems for Stretching Limited Irrigation Supplies

Abstract

Declining groundwater levels in irrigated areas of the Great Plains may require a shift to limited irrigation or a return to dryland production. 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 farming, and limited and full irrigation. The limited irrigation allocation 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 practices for fertilizer, herbicide, variety selection, and pest management were modified as improved varieties, technology, and products became available. In the winter wheat (WW)‐corn (C)‐soybean (SB) rotation, 6‐yr average yields for winter wheat were 33, 64, and 65 bu/acre for dryland, limited, and full irrigation, respectively. Average corn yields were 90,161, and 188 bu/acre for the three moisture regimes, whereas soybean average yields were 29, 52, and 59 bu/acre. The limited irrigation yields were 99, 86, and 88% of fully irrigated yields for winter wheat, 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 winter wheat. Poor stand establishment in heavy residue and chlorosis were problems in grain sorghum culture, which may limit its competitiveness with corn in this area. This research demonstrated that minimum tillage cropping systems and limited irrigation have possibilities for maintaining acceptable yield levels of corn, winter wheat, and soybean in areas of declining irrigation water. Research Question Declining groundwater levels in irrigated areas of the Great Plains may require a shift to limited irrigation or a return to dryland crop production. Advances in dryland agriculture through the use of herbicide, residue management, and no‐till farming have improved dryland grain yield stability. Dryland grain yields and profitability are lower and not as stable as those with irrigated agriculture. Crop production data are needed to assist in decisionmaking in regions that may experience a transition from full irrigation to limited irrigation or dryland agriculture. The techniques of moisture conservation required for successful dryland farming plus the appropriate timing of limited irrigation must be researched to determine the production potential of different cropping systems. Such systems might extend aquifer life and provide smoother transition from intensive irrigation to dryland or limited irrigation in the central Great Plains. The primary objective of this study was to determine the yields that could be attained with the application of limited irrigation in minimum tillage cropping systems with four crops common to the Great Plains: winter wheat, corn, grain sorghum, and soybean. Literature Summary Numerous studies throughout the Great Plains have shown that the substitution of herbicides for tillage in dryland farming systems conserves soil and more of the natural precipitation for crop growth. Advances have been made in winter wheat‐grain sorghum‐fallow or winter wheat‐corn‐fallow systems. Irrigation research has shown that some stress can occur in the vegetative period and not significantly reduce yields. Applying limited amounts of irrigation water during the reproductive and early grain fill stages provided the greatest return for the irrigation input. The production potential of limited irrigation in minimum tillage cropping systems adaptable to the Great Plains has not been investigated. Study Description The experiment was conducted for 10 yr at North Platte, NE, on a Cozad silt loam soil using cropping systems that included winter wheat, corn, soybean, and grain sorghum. Three adjacent 6‐acre blocks of land were used for three water treatments—dryland, limited irrigation (6 in./growing season), and full irrigation. Limited irrigation was applied during the beginning reproductive to early grain filling period. Cropping systems included winter wheat‐corn‐soybean, winter wheat‐soybean, continuous winter wheat, continuous corn, corn‐soybean, winter wheat‐corn‐sorghum, and winter wheat‐grain sorghum‐soybean. Changes were made in cultural practices, or the rotations were dropped/changed if cultural problems could not be corrected or if it appeared the rotations would have limited farmer acceptance. Full scientific article from which this summary was written begins on page 520 of this issue. Applied Questions How did the limited irrigated yields compare with dryland and full irrigation? The limited irrigated yields for winter wheat, corn, and soybean were 99, 86, and 88% of the fully irrigated yields (Fig. ). The marginal return from the 6 in. of limited irrigation compared with dryland was 10.9 bu/in. for corn, 5.9 bu/in. for winter wheat, 4.3 bu/in. for soybean, and 3.1 bu/in. for sorghum (Table ). This research established a data base that can be used to evaluate the economic potential of a shift from full to limited irrigation in areas of declining groundwater. What did we learn about cultural practice problems for these crops while conducting the research? Grain yields from crops grown under dryland, limited irrigation, and full irrigation in continuous corn and the winter wheat‐corn‐soybean rotation. image Machinery modifications were required for planting winter wheat into heavy crop residue and hard, dry, untilled soil. A limited amount of tillage, closer row spacing, and higher winter wheat seeding rates helped overcome wheat stand problems caused by toxicity of soybean residue. Winter wheat following soybean showed significant yield increases from N fertilization. The N fertilization produced higher levels of soil nitrate after wheat, but this reduced N fertilizer for crops following wheat. The row‐crop planter was modified to remove an 8 in. strip of residue above the seed row. This improved emergence and stand uniformity. Herbicide management was a challenge because of the high pH. The introduction of new herbicides during the experiment helped overcome problems with crop damage or loss in the early years. Variety selection to match the yield potential with available soil moisture while tolerating the high pH conditions, especially for soybean and grain sorghum, was a challenge. Standard fertilization practices based on University of Nebraska correlated/calibrated soil testing provided optimum fertilizer management. Recommendation The limited irrigation cropping systems research demonstrated that crop rotations including winter wheat, corn, grain sorghum, and soybean have possibilities for stretching limited water supplies and extending aquifer life in areas of declining groundwater. Cropping systems using these crops conserve natural precipitation using no‐till farming techniques and crop residue management. Applying the limited amount of irrigation water available during the crop reproductive and early grain fill periods proved to be feasible in this experiment. Timely management of all operations was critical for the system to work. Marginal grain yield increases from water applied under limited and full irrigation. Crop Limited Full irrigation ‐‐‐‐‐‐‐‐‐‐bu/in.‐‐‐‐‐‐‐‐‐‐ Continuous corn 12.3 4.5 Corn in winter wheat‐corn‐soybean 10.9 3.8 Soybean in winter wheat‐corn‐soybean 4.3 1.3 Winter wheat in winter wheat‐corn‐soybean 5.9 0.6 Sorghum 3.1 0.0