Impact of defoliation on yield of group IV soybeans in Mississippi

- Yield loss from defoliation was greater during R3 and R5 than R6.
- Defoliation in the top of the canopy reduced yield greater than defoliation in the lower canopy during R3 and R5.
- Soybeans are more susceptible to yield loss from high levels (>63%) defoliation at R3 and R5.
- Dynamic thresholds can be estimated based on value of the crop per hectare, and the cost of the insecticide application.

Field experiments were conducted during 2009 and 2010 to evaluate the effects of defoliation on maturity group IV soybeans, Glycine max (L.) Merr., grown in Mississippi. During each year, two locations were planted with maturity group IV soybeans that were subjected to various levels of defoliation during R3, R5, and R6 growth stages. Soybeans were subjected to various levels of defoliation within the upper 50% of the plant canopy, lower 50% of the plant canopy, and whole-plant canopy. There was greater yield loss from defoliation occurring in the upper plant canopy compared with the lower plant canopy during R3 and R5 stages, but no difference between canopy regions during R6 stage. Yield loss from whole plant defoliation was greater than upper or lower canopy defoliation. Results confirmed that soybeans during R3 and R5 stages are more susceptible to yield loss than during R6. However, yield losses were not significantly different between R 3 and R5 until defoliation exceeded 63%. Dynamic economic injury levels were determined for each growth stage based on yield loss equations, value of the crop, and cost of control and can be used as a basis for developing action thresholds in high-yielding soybean production environments.