EARLY-MATURING SOYBEAN CROPPING SYSTEM .2. GROWTH AND DEVELOPMENT RESPONSES TO ENVIRONMENTAL-CONDITIONS

Production of early-planted, early-maturing soybean [Glycine max (L.) Merrill] cultivars has gained popularity in the southeastern USA in re cent years, primarily as a drought-avoidance mechanism. In seasons wit h adequate rainfall, however, early planting of such cultivars may not be essential for competitive yields. Our objective was to identify ke y environmental constraints to the growth and development of early-mat uring soybean in the southern region. Soybean cultivars from Maturity Groups (MG) 00 through IV were planted in late April, mid-May, early J une, and late June in 1990 through 1993 in 0.38-m rows on a well-drain ed Maury silt loam soil (fine, mixed, mesic Typic Paleudalf). Canopy c losure, plant height, and plant mass per unit area were recorded at bo th R1 and R5. In addition, crop growth rate from R1 to R5 was calculat ed. Cool vegetative-stage temperatures suppressed vegetative growth, p articularly of early-maturing cultivars and for early planting dates. Crop growth rate during pod-set was not consistently associated with y ield. Canopy closure at R1 of early-maturing cultivars was more respon sive to delayed planting than was that of later maturing cultivars. Ac ross years and planting dates, canopy closure at R5 of MG 00 to I cult ivars generally exceeded 90%. Early planting may be disadvantage for e arly-maturing cultivars in seasons with favorable rainfall patterns, p articularly if canopy development is inhibited by cool temperatures du ring vegetative growth. If moisture is not limiting, delayed planting of early-maturing cultivars may be advantageous.