Important Change to North Carolina Soybean Defoliation Thresholds After Beginning Flowering (R1)
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Collapse ▲Soybean can tolerate considerable defoliation before it loses yield. Common major defoliating pest species in NC include bean leaf beetle, soybean looper, and velvetbean caterpillar. Prior to flowering, defoliation thresholds range from 30% to 40% and after flowering begins, defoliation thresholds range from 15% to 25% In the Southern US.
The historic NC reproductive-stage defoliation threshold (15%) is the most conservative in the southern US and is not based on research conducted in North Carolina. Our current defoliation threshold (15%) begins at two weeks prior to flowering and continues through reproductive development to R6.5 .
We are changing the defoliation threshold for full-season soybeans in reproductive development based on recent research conducted in the NC to:
After bloom: 25% of the canopy- full-season soybean (planted prior to June 1)
Defoliation thresholds will remain the same in vegetative growth stages and for reproductive stages in double crop beans
Vegetative Growth Stages: 30% of the canopy- full-season soybean (planted prior to June 1)
After bloom: 15% of the canopy- double-cropped soybean (planted after May 31)
Our rationale is based on NC research findings that full-season soybeans can tolerate up to 66% defoliation in the reproductive growth stages without experiencing a yield loss. Double-cropped soybeans were more sensitive, losing yield between 33% to 100% defoliation, depending on conditions.
However, our updated threshold of 25% defoliation is still conservative compared to the research maximum of 66%. This accounts for variability in field conditions, pest pressure, and grower risk tolerance. Thresholds are set to provide a margin of safety to ensure yield protection across a range of environments and management practices.