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Early-Maturing Soybean Cropping System: II. Growth and Development Responses to Environmental Conditions

Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091.

^* Corresponding author (lgrabau{at}ca.uky.edu).

Production of early-planted, early-maturing soybean [Glycine max (L.) Merrill] cultivars has gained popularity in the southeastern USA in recent years, primarily as a drought-avoidance mechanism. In seasons with adequate rainfall, however, early planting of such cultivars may not be essential for competitive yields. Our objective was to identify key environmental constraints to the growth and development of early-maturing soybean in the southern region. Soybean cultivars from Maturity Groups (MG) 00 through IV were planted in late April, mid-May, early June, and late June in 1990 through 1993 in 0.38-m rows on a well-drained Maury silt loam soil (fine, mixed, mesic Typic Paleudalf). Canopy closure, plant height, and plant mass per unit area were recorded at both R1 and R5. In addition, crop growth rate from R1 to R5 was calculated. Cool vegetative-stage temperatures suppressed vegetative growth, particularly of early-maturing cultivars and for early planting dates. Crop growth rate during pod-set was not consistently associated with yield. Canopy closure at R1 of early-maturing cultivars was more responsive to delayed planting than was that of later maturing cultivars. Across years and planting dates, canopy closure at R5 of MG 00 to I cultivars generally fell below 90%, while that of MG II to IV cultivars generally exceeded 90%. Early planting may be a disadvantage for early-maturing cultivars in seasons with favorable rainfall patterns, particularly if canopy development is inhibited by cool temperatures during vegetative growth. If moisture is not limiting, delayed planting of early-maturing cultivars may be advantageous.

Received for publication June 16, 1995.

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