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

The effects of soybean seeding rates on evapotranspiration (ETc) and grassand alfalfa-reference crop coefficients (Kco and Kcr) 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). Kco and Kcr values were developed for each soybean growth stage. The seasonal ETc 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, ETc 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 ETc were observed across irrigation levels than seeding rates, indicating that water availability has a greater impact on ETc than seeding rates. Kc values were strongly correlated with days after emergence (DAE) and growing degree days (GDD). The seasonal maximum Kco varied among seeding rates and was observed between GDDs of 924 and 1200oC, corresponding to the R5 (beginning seed) and R6 (full seed) growth stages