Investigating soybean (Glycine max L.) responses to irrigation on a large-scale farm in the humid climate of the Mississippi Delta region

The shallow Lower Mississippi River Valley Alluvial Aquifer, which supports irrigated agriculture in the Lower Mississippi Delta (LMD) region, is fast depleting from unsustainable water extractions for irrigations. The survival of irrigated agriculture in the region today hinges on enhancing the irrigation use efficiencies of the water pumped out. Furrow irrigation practices (surface or flood irrigation) dominate the LMD region's irrigated agriculture scenario. We investigated soybean productivity in response to irrigations applied through every furrow (FI), applied through alternate furrow (AFI), and rainfed (RF, no irrigation). Approximately half the volume of water applied in FI was applied in the AFI. The experiments were conducted in 2016, 2018, and 2020, which constituted the soybean phases of a corn-soybean rotation trial conducted on a clay soil in farm-scale plots (15 ha). The plots were equipped with eddy covariance systems for quantifying crop water use (ET, evapotranspiration). There was no appreciable difference in soybean grain yield between FI and AFI, but RF yielded significantly lower than FI and AFI. Leaf area index was also significantly lower in RF compared to FI and AFI. Across the three years, the average reduction in soybean yield was only 2% in AFI, while it was 24% in RF compared to FI. Average grain yields were 4507, 4413, and 3422 kg ha⁻¹; seasonal ET were 549, 562, and 527 mm; and water use efficiencies (WUE) were 8, 8, and 7 kg ha⁻¹ mm⁻¹ in FI, AFI, and RF, respectively. This large farm-scale study demonstrated that grain yields from irrigating soybean through alternate furrows were comparable to irrigating through every furrow, thus saving about half the water pumped out of the aquifer. This unique study was conducted in farm-scale fields; as such, the results obtained directly apply to a farm environment, so they are ready for recommendation to soybean farmers for adoption without further field trials.