Soybean crop coefficients under different seeding rates and full and limited irrigation and rainfed management

The effects of soybean seeding rates on evapotranspiration (ET_c) and grass- and alfalfa-reference crop coefficients (K_co and K_cr) were investigated under five seeding rates (185,250, 247,000, 308,750, 370,500 and 432,250 seeds ha⁻¹) (62, 82, 103, 124 and 144% of the recommended seeding rate of 300,000 seeds ha⁻¹ in the experimental region) and different irrigation strategies (i.e. full irrigation treatment [FIT], limited irrigation of 75% FIT and 50% FIT and rainfed treatment). K_co and K_cr values were developed for each soybean growth stage. The seasonal ET_c ranged from 460 mm for rainfed to 489 mm for FIT in 2014 under the same seeding rate of 247,000 seeds ha⁻¹. In 2015, ET_c ranged from 308 mm for rainfed conditions under the highest seeding rate (432,250 seeds ha⁻¹) to 395 mm for FIT conditions under 308,750 seeds ha⁻¹. Greater differences in weekly ET_c were observed across irrigation levels than seeding rates, indicating that water availability has a greater impact on ET_c than seeding rates. K_c values were strongly correlated with days after emergence (DAE) and growing degree days (GDD). The seasonal maximum K_co varied among seeding rates and was observed between GDDs of 924 and 1200°C, corresponding to the R5 (beginning seed) and R6 (full seed) growth stages.