Corn, Sorghum, and Soybean Response to Irrigation in the Mississippi River Alluvial Plain

The most agronomically efficient use of irrigation water is for those crops that give the greatest response. In the Mississippi River alluvial plain, the primary irrigated crop is soybean [Glycine max (L.) Merr.], but the response of soybean to irrigation has not been compared to that of other crops. Irrigated and nonirrigated experiments were conducted from 1984 through 1987 on Tunica clay (clayey over loamy, montmorillonitic, nonacid, thermic Vertic Haplaquept) to determine the effect of irrigation on field-grown corn (Zea mays L.), sorghum [Sorghum bicolor (L.) Moenchl, and soybean yield and yield components. Shifts in weed species composition resulting from continuous monocropping with these crops also were quantified. Irrigation was applied from beginning bloom to near matarity of each crop whenever soil water potential at the 30-cm soil depth averaged about —70 kPa. Irrigation did not consistently affect weed cover in any of the crops. Weed level differences among crops resulted from different weed control programs for each continuous cropping system. Differences between average seed yields of irrigated (I) and nonirrigated (NI) corn, sorghum, and soybean 2886, 694, and 1574 kg ha⁻¹, respectively. Sorghum produced the most stable nonirrigated yield and the smallest increase in monetary return from irrigation. Differences between I and NI corn and soybean yields were associated with increased number of seed. Smaller sorghum yield differences were associated with differences in seed weight or a combination of differences in seed weight and number of seed. Across the 4 yr, irrigation of corn and soybean produced nearly equal increases in gross income per unit of land area, but irrigation efficiency for soybean was lower because achieving the increased return from irrigation required nearly twice as much water for soybean as for corn.