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Soybean [Glycine max (L.) Merr.] yield, water use efficiency (IWUE), actual evapotranspiration (ETa), crop water productivity (CWP) and crop coefficients were assessed for five soybean seeding rates of 185,250, 247,000, 308,750, 370,500 and 432,250 seeds per hectare and three irrigation strategies i.e. full irrigation treatment (FIT), 75% FIT and 50% FIT and rainfed treatment under sub-surface drip irrigation. The field study was conducted at University of Nebraska-Lincoln, South Central Agricultural Laboratory (SCAL) during 2014 and 2015 growing seasons. Soybean yield varied from 4.44 to 4.80 tons ha-1 for rainfed treatments and from 4.22 to 4.90 tons ha-1 for irrigated treatments in 2014. In 2015, soybean yield were in range of 3.49 to 3.87 tons ha-1 under rainfed treatment and 4.00 to 4.73 tons ha-1 under irrigated treatments. There was no significant difference between 75% FIT and FIT in terms of grain yield. The highest IWUE was highest (0.96 kg m-3) for 247,000 seeds ha-1 under 75% FIT in 2014 and in 2015, highest IWUE under 50% FIT, 75% FIT and FIT were 1.49, 1.14 and 1.06 kg m-3 for 308,750 seeds ha-1. Seasonal ETa ranged from 459.7 mm for rainfed treatment to 489.1 mm for FIT, respectively, under the same seeding rate of 247,000 seeds ha-1 in 2014 and from 307.6 mm for rainfed treatment under highest seeding rate of 432,250 seeds ha-1 to 394.8 mm for FIT under 308,750 seeds ha-1 in 2015. Seasonal ETa exhibited a linear response to irrigation amounts and seed yield during both growing seasons although the response was weak (R2=0.25 in 2014 and R2=0.42 in 2015) when pooled for all seeding rates. The highest weekly ETa of 61.8 mm and 43.7 mm when averaged for all irrigation treatments was recorded for seeding rate of 308,750 seeds ha-1 in 2014 and 2015, respectively. CWUE was greater in 2015 (0.95 to 1.25 kg m-3) because of lesser precipitation than in 2014 (0.82 to 0.95 kg m-3). The highest CWUE value of 0.93 kg m-3 occurred for 308,750 seeds ha-1 in both 75% FIT and FIT in 2014 while in 2015 highest CWUE of 1.25 kg m-3 was measured for FIT under 432,250 seeds ha-1. The range of Kco and Kcr was 0.49 to 1.39 and 0.36 to 1.15, respectively, in 2014 while in 2015 the range for Kco was 0.19 to 1.34 and Kcr was 0.14 to 1.07. Kc values showed variability due to irrigation treatments in both years. The maximum Kc values were observed during R5-R6 stage in 2014 and at R6-R7 stage in 2015.
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