EARLY-MATURING SOYBEAN CROPPING SYSTEM .1. YIELD RESPONSES TO PLANTING DATE

An early-planted, early-maturing soybean [Glycine max (L.) Merrill] cr opping system has been quite successful across the southeastern USA. T his cropping system was intended to use early planting to improve yiel d by avoiding late season drought stress. However, producers in this r egion use planting dates ranging from late April through late June. Ou r objective was to characterize the yield response of early-maturing c ultivars to environmental conditions over a range of planting dates. F ield studies were conducted on a well-drained Maury silt loam (fine, m ixed, mesic Typic Paleudalf) in 1990 through 1993 using cultivars from Maturity Group (MG) 00 through IV planted in late April, mid-May, ear ly June, and late June each year. Modest moisture stress was observed over the years tested; however, temperatures during soybean growth sta ges depended strongly on planting dates. When averaged across years an d cultivars, yields were similar for the first three planting dates. B est overall yields were obtained using the MG IV cultivar for late Apr il and mid-Map planting; however, its advantage over MG II and III cul tivars diminished for the early lune planting date and disappeared for the late June planting date. Early-maturing cultivars (MG 00 to I) we re less likely to achieve competitive yield levels. Lower yields of ea rly-maturing cultivars for the late April planting date were associate d with low temperatures during vegetative growth and with high tempera tures during seed-fill. In the absence of pronounced moisture stress, low or high temperatures may become the key environmental factors limi ting yield. Early planting, a centerpiece of previous successful early -maturing soybean cropping systems in the southeastern USA, appears to be less important under more favorable moisture conditions.