Method to Use Crop Growth Models to Estimate Potential Return for Variablerate Management in Soybeans

The objective of this study was to use a soybean crop model to estimate the difference in net return betweenvariablerate and uniformrate management for soybean plant population and variety selection decisions. TheCROPGROSoybean model was calibrated to fit three seasons of historical yield data in 77 grids within a 20ha field infestedwith soybean cyst nematodes (SCN) in central Iowa. Procedures were developed to compute the net return and breakevencost for variable plant population density and variety selection for SCN management using 34 years of historical weatherdata. Implementing the best population (VRX) for each year produced higher net returns compared to using the 34yearaverage optimum rate (VRA) or using uniform management (UM). Achieving maximum possible net return under VRX maynot be possible on a yearly basis due to uncertainties in weather condition. Riskaverse farmers, however, may opt to use therealistic 34year average optimum rate (VRA) that provides favorable net return over the long term. Procedures were alsodeveloped to determine gross profit and breakeven costs for switching SCNresistant and susceptible varieties within anSCNinfested field. Using an SCNresistant variety across the entire field resulted in significant net returns over that of asusceptible variety, and there appeared to be no economic advantage for variable variety selection compared to planting auniform resistant variety across the field.