Effects of Soybean Plant Population on Yield Loss From Defoliation.

Plant densities in Mid-South U.S. soybean, Glycine max (L.) Merr., fields can vary greatly due to a wide range of factors, although soybean yields are generally insensitive to variations in density. Currently, it is unknown if yield loss from insect-related defoliation varies across different soybean stand densities. Soybean was planted in Starkville and Stoneville, MS, in 2016 and 2017 at five seeding rates ranging from 123,500 to 420,070 seed/ha in 74,130 seed/ha increments. Each seeding rate contained a nondefoliated plot and a plot that was defoliated 67% at the R1 growth stage. Defoliated plants had a greater leaf expansion rate from R1 to R3 than nondefoliated plants. Defoliation reduced yield where plant densities were <192,800 plants/ha, but greater densities were not affected. Reduced yield in defoliated plots when compared with nondefoliated plots at equivalent R3 leaf area index values indicated that some resources were used to replace the removed leaf area instead of seed production. These results suggest that fields with substandard plant densities might benefit from a reduced treatment threshold for defoliating pests.

Effects of Soybean Planting Date on Yield Loss From Defoliation.

Soybean, Glycine max (L.) Merr., is planted during 3.5-4 mo across the Mid-South United States. Currently, no information exists regarding the effects of planting date on soybean yield loss from early season defoliation. In 2015 and 2016, to evaluate the effects of planting date on yield loss from defoliation, soybean were planted in field plots 2 wk apart from early April to mid-June, for a total of six planting dates. Each planting date included a nondefoliated control and a 100% defoliation treatment where leaves were manually excised at the V4 growth stage. Mean yield loss from defoliation varied across planting dates, with mid-April plantings having the least amount yield reduction, 573 kg/ha, and early-June plantings having the greatest yield reduction, 904 kg/ha. Percent yield reduction from defoliation increased as planting was delayed, suggesting that defoliation thresholds might need adjustment based on planting date and yield potential. However, more research is needed at lower levels of defoliation to accurately delineate such thresholds.