Sean Malone, D. Ames Herbert Jr., and David Holshouser. Virginia Tech - Tidewater AREC, 6321 Holland Rd., Suffolk, VA 23437
Previous research on full-season soybean [Glycine max (L.) Merrill], which is a single crop planted early in the season, indicates that leaf area index (LAI) values of at least 3.5 to 4.0 in the reproductive stages are required for maximum potential yield. We hypothesized that double-crop soybean, which is planted late into harvested small grain fields, would be more sensitive to defoliation than full-season soybean. From 15 manual defoliation experiments over three years, significant linear decreases in yield occurred in both full-season and double-crop soybean when LAI values were below 3.5 to 4.0 by developmental stages R4 to R5, while yields usually reached a plateau at higher LAI levels. Average yield loss was 769 ± 319 kg ha-1 for each unit decrease in LAI below the plateau; average maximum yield was 3484 ± 735 kg ha-1. Therefore, full-season and double-crop soybean that maintain LAI values above the 3.5 to 4.0 critical level by mid-reproductive developmental stages should be tolerant of defoliation.
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