Method to use crop growth models to estimate potential return for variable–rate management in soybeans

The objective of this study was to use a soybean crop model to estimate the difference in net return between variable–rate and uniform–rate management for soybean plant population and variety selection decisions. The CROPGRO–Soybean model was calibrated to fit three seasons of historical yield data in 77 grids within a 20–ha field infested with soybean cyst nematodes (SCN) in central Iowa. Procedures were developed to compute the net return and break–even cost for variable plant population density and variety selection for SCN management using 34 years of historical weather data. Implementing the best population (VRX) for each year produced higher net returns compared to using the 34–year average optimum rate (VRA) or using uniform management (UM). Achieving maximum possible net return under VRX may not be possible on a yearly basis due to uncertainties in weather condition. Risk–averse farmers, however, may opt to use the realistic 34–year average optimum rate (VRA) that provides favorable net return over the long term. Procedures were also developed to determine gross profit and break–even costs for switching SCN–resistant and susceptible varieties within an SCN–infested field. Using an SCN–resistant variety across the entire field resulted in significant net returns over that of a susceptible variety, and there appeared to be no economic advantage for variable variety selection compared to planting a uniform resistant variety across the field