Developing guidelines for replanting grain sorghum .3. Using a plant growth model to determine replanting options

The adaptability of grain sorghum [Sorghum bicolor (L.) Moench] complicates decision making regarding the replacement of damaged stands. Our objective was to construct replanting guidelines for a broad range of situations by using a sorghum growth model to project yields for both original and replanted stands. Sorghum yields were simulated for 33 seasons for every combination of seven planting dates, six plant populations, and three hybrid maturity classes at three Kansas sites representative of Midwest sorghum producing areas. The outputs were tabulated to obtain yield distributions for both original and replanted crops. Net returns for replanting were calculated from yield differences, input costs, and harvest prices. Increases in both mean yields and net returns were associated with greater variability. The choices of replant timing, hybrid maturity, and planting density therefore depend on the grower's risk preferences. In western Kansas, an area with soils and climate representative of most of the Great Plains, risk-neutral producers should replant with an early-maturity hybrid at 50 000 plants ha(-1) if the original population declines to less than 20 000 plants ha(-1). In eastern Kansas, an area representing western Corn Belt sorghum production, replanting with a medium-maturity hybrid at 200 000 plants ha(-1) is a good option when a medium- or late-maturity hybrid falls below 20 000 plants ha(-1) or an early hybrid drops beneath 100 000 plants ha(-1).