PLANTING DATE AND SOYBEAN YIELD - EVALUATION OF ENVIRONMENTAL-EFFECTS WITH A CROP SIMULATION-MODEL - SOYGRO

Planting date is an important factor influencing soybean [Glycine max (L.) Merrill] yield. The effects of planting date on yield were investigated using SOYGRO V5.41, a dynamic, process-oriented crop growth computer model, developed at the University of Florida. SOYGRO calculates daily growth, development and yield of a soybean community. Simulated yields were computed for nine planting dates from 1 May to 30 June, two cultivars (Williams-maturity group III and Essex-maturity group V) at two row spacings (0.76 and 0.18 m) and with natural rainfall or without water stress. Seventeen years of weather data from Lexington, KY (38-degrees-N latitude) were used. The average yield across 17 years was taken as the average response to planting date. The model accurately predicted observed yield responses for a planting date experiment at Lexington (1967-1968) and the combined general responses of other planting date experiments from several locations in Kentucky. The decreases in simulated yields from June plantings of 'Williams', but not 'Essex', were reduced, but not eliminated, when there was no water stress. Increasing maximum and minimum temperature by 20% during reproductive growth decreased the simulated yield of 'Williams' in June planting dates but increased the yield of Essex. Increasing insolation by 20% increased simulated yield of both cultivars. The results indicate that lower insolation during reproductive growth is the primary cause of reduced yields from delayed plantings; low temperature contributed to lower yields only for the late maturing cultivar.