Yield Benefits from Combined Applications of Foliar Fungicides and Insecticides on Soybean.

Soybean producers are looking for ways to protect soybean (Glycine max (L.) Merr) yields from a range of diseases and pests in Iowa. Combining a foliar fungicide with an insecticide can save costs by avoiding extra trips across fields and increase yield through possible synergistic effects of using two chemicals in one application. However, the economic benefits of the combined use of fungicides and insecticides are not studied extensively and unneeded applications of either chemicals can lead to resistance. This study evaluated the joint use of several fungicides and insecticides at R1 through R3 soybean growth stage. Two groups of on-farm replicated strip trials were established across Iowa between 2006 and 2016. One group had untreated control compared to the insecticide and fungicide mix. The other group had three treatments: untreated control, fungicide alone and the insecticide and fungicide mix. Farmers used FRAC 3 and 11 fungicides and pyrethroid and/or neonicotinoids insecticides. Georeferenced yield data were collected using combines equipped with GPS enabled yield monitors.
Across 25 two-treatment trials, the fungicide and insecticide mix produced a yield response of 250 kg ha-1, with about half of locations having significant yield responses. Across 25 three-treatment trials, the yield response to the mixed treatment was about 190 kg ha-1 (90% CI from 120 to 245), but the mean yield response to the fungicide alone was about 130 kg ha-1 (90% CI of 95 and 180). The insecticides significantly increased yields at several locations with high aphid pressure in 2013 and 2014.
Because of the relatively low cost of insecticide, the use of both chemicals in single application could be profitable for farmers. However, this study showed that most of yield responses were due to fungicides and the insurance applications of insecticides on top of the fungicides did not increase yield significantly.