North Carolina soybean defoliation thresholds are very low and not in line with other thresholds from the southeastern US. Our thresholds are based on work with a full-season soybean with a determinate growth habit over 50 years ago. Obviously, this is not applicable to today’s production systems. This proposal seeks to make a first step at evaluating these thresholds in production systems that are more typical of growers in our state. We propose to 1) see how much defoliation it takes to affect yield in double cropped soybean at different growth stages and 2) compare defoliation thresholds between full-season and double-cropped soybeans. These might prevent potentially costly insecticide sprays for growers if thresholds can be increased or lead to increased yield and profit for growers if thresholds can be decreased.

1. When should defoliation thresholds change in double cropped soybean?
2. Does defoliation at R5 impact full-season soybean differently than double cropped soybean?

Threshold changes will not be made after a single year of experiments. However, if we are able to accomplish the objectives after two years and results indicate that threshold changes are needed, we will modify the vegetative thresholds for North Carolina soybean. These results will be posted at the NCCE soybean portal (soybeans.ces.ncsu.edu) and will be disseminated at field days and county winter meetings. The NCPSA will be acknowledged as the funding source.

North Carolina soybean defoliation thresholds are very low and not in line with other thresholds from the southeastern US. We recommend that growers treat once defoliation reaches 30% during the vegetative stages up until two weeks prior to bloom. Then we recommend that growers treat at 15% defoliation until R6.5 to R7. This is most similar to Georgia, South Carolina, and Virginia thresholds, which are 30% pre-bloom and 30% once blooming. Virginia recommends doubling, and in some cases tripling the threshold at R6. Alabama thresholds are even higher at 35% pre-bloom and 20% once blooming. In addition, they have thresholds based on defoliating insect numbers.
If research supports that defoliation thresholds in North Carolina can be raised, growers stand to benefit by waiting longer to spray. This could result in significant cost-savings by reducing the number of insecticide sprays and losses from driving over soybeans. However, changes should be approached with caution as previous research suggests that defoliation can impact soybean greatly depending on planting date, plant density, and yield potential. Therefore, more focus will initially be placed on double-cropped soybean, as these planting dates are more at risk for yield loss from defoliation and may not have an adequate foliage canopy for compensation.