Soybean cultivar differences on light interception and leaf area index during seed filling

Light interception and leaf area criteria to maintain optimum soybean [Glycine max (L.) Merr.] yield during the last half of the seed filling period have been developed since this is the period of most frequent infestations by defoliating insects in the southeastern USA. Because these criteria were developed for only one cultivar, Centennial, they would have greater applicability if shown to be applicable across a range of cultivars. The objective of this research was to determine if these previous criteria are applicable across a range of commercial soybean cultivars. The study was conducted at the Ben Hur Research Farm near Baton Rouge, LA (30degreesN lat), on a Commerce silt loam soil (fine-silty, mixed, nonacid, thermic, Aeric, Fluvaquent). Planting dates were 3 June 1997 and 30 June 1998. The experimental treatments consisted of a factorial combination of 10 soybean cultivars and three leaf defoliation treatments: a nondefoliated control, partial defoliation at the temporal midpoint of seed filling, and total defoliation at the temporal three-fourths point of seed filling. Leaf area index (LAI) and canopy light interception (LI) were measured after defoliation treatments. Grain yield was determined at maturity by machine harvest. Partial defoliation at mid-seed filling significantly (P < 0.05) reduced LAI, LI, and yield in 8 out of 10 cultivars in at least 1 yr of the study. Total defoliation at the three-fourths point of seed filling also reduced yields for almost every cultivar-year treatment combination. Our results tended to support the original criteria related to LI and leaf area criteria for maintenance of optimal yield.