Effects of subsurface drip-irrigated soybean seeding rates on grain yield, evapotranspiration and water productivity under limited and full irrigation and rainfed conditions

There is lack of research, data and knowledge that can be transferable to producers, their advisors and managers in terms of dynamic relationships of soybean [Glycine max (L.) Merr.] yield, crop evapotranspiration (ETc), crop water productivity (CWP) with respect to irrigation water applied, seeding rate, and irrigation vs. ETc relationships. These dynamics were investigated relative to five seeding rates (185,250; 247,000; 308,750; 370,500 and 432,250 seeds ha−1) under full irrigation treatment (FIT), limited irrigation (75% FIT and 50% FIT), and rainfed. Rainfed treatment had the highest soil-water depletion, followed by 50% FIT, 75% FIT and FIT. No clear trend of soil-water depletion was observed due to seeding rates. In 75% FIT and FIT, highest yield was observed with 308,750 seeds ha−1 seeding rate. Yield plateaued at 4.3 tons ha−1 and no further yield increase was observed with increasing seeding rate beyond 308,750 seeds ha−1. While ETc differed significantly (p < 0.05) due to irrigation treatments and seeding rates, ETc was affected more by the irrigation levels than the seeding rates. ETc ranged from 460 mm for rainfed treatment to 489 mm for FIT under the lowest seeding rate (247,000 seeds ha−1) in 2014; and from 308 mm for rainfed under highest seeding rate (432,250 seeds ha−1) to 395 mm for FIT under 308,750 seeds ha−1 in 2015. ETc exhibited a linear response to irrigation and yield. Irrigation vs. ETc slopes indicate that for every 25.4 mm increase in irrigation application, ETc can increase between 5.8 mm and 17.3 mm, depending on the seeding rates. The highest weekly ETc of 62 mm in 2014 and 44 mm in 2015 was recorded for the same seeding rate of 308,750 seeds ha−1. IWP was highest (0.96 kg m−3) for 247,000 seeds ha−1 under 75% FIT in both seasons. The highest CWP of 0.93 kg m−3 was obtained with the 308,750 seeds ha−1 in both 75% FIT and FIT in 2014 while the highest CWP of 1.25 kg m−3 was measured for FIT under 432,250 seeds ha−1 in 2015. Further long-term research is suggested to investigate the potential differences in physiological and phenological parameters, growth, ETc, water productivity, yield, and yield response factors under different seeding rates for different soybean varieties under different irrigation levels and methods.