Incidence and Causes of Soybean Plant Attrition.

Research supported by the Tennessee Soybean Promotion Board to document and determine the causes of soybean plant attrition suggests that producers are obtaining current soybean yields with less than realized plant populations.  While seedling diseases and insects have been identified as causes of early-season stand losses, plant attrition throughout the season has been largely unrealized, but often results in significantly reduced stands at harvest.  A common assumption has been that plants that have survived seedling diseases and early-season insects and appear healthy and vigorous at early vegetative stages (when most stand counts are taken and reported) will survive throughout the season and contribute to final yields, but field studies at the University of Tennessee Research and Education Center at Milan have revealed attrition rates ranging from 0-30% when early-season stand counts are compared to final stand counts.  Since soybean plants compensate for reduced stands with increased branching, soybean yields among differing final plant populations are often similar and this loss of plants throughout the season is seldom realized.  Although soybean yields may not reflect the loss of plants, soybean plant attrition reduces profitability because seed utilization has not been optimized.  This is particularly evident in higher planting populations, where plant attrition rates tend to be high, likely resulting from self-thinning due to intraspecific competition.