Relative Soybean Response to Aminocyclopyrachlor, 2,4-D, Dicamba, and Aminopyralid

This research compared the visual symptomology of four auxin herbicides applied at three rates to simulate tank contamination on non-auxin tolerant soybeans. The herbicides examined were aminocyclopyrachlor (ACP), 2,4-D, dicamba, and aminopyralid. The standard labeled rates (SLR) used to base the application rates were 0.12 kg ae ha^-1 for ACP and aminopyralid while dicamba and 2,4-D used 1.12 kg ae ha^-1. These four herbicides exhibit similar symptoms on soybeans which is problematic when diagnosing soybean response. One goal of this project was to obtain high-quality images of the four herbicide’s effect on soybeans, with the hope of being able to discern the herbicides effects on soybeans solely from visual symptoms. Each herbicide was applied at three different rates: 0.1X, 0.01X, and 0.001X of the SLR. The application was made at the R1 soybean stage. A randomized complete block design with three replications was utilized. Border rows for each plot were inspected for injury to assess potential drift between plots. The plots were visually evaluated for herbicide response at 3, 7, 14, and 21 days after treatment (DAT). Two trifoliate leaves were collected during each evaluation for future chemical analysis (data not shown). At 21 DAT, all 0.1X treatments had greater than 20% crop injury. The 0.001X rate for each herbicide exhibited different symptomology when compared to 0.1X rate. Leaf cupping and galls were more prominent on soybeans at lower herbicide dosages. Increasing the herbicide dosage changed symptomology from leaf cupping and galls to necrosis, leaf curling, and epinasty. There were no profoundly distinct visual differences between herbicides. Soybean yield and herbicide dosage were inversely related, and each herbicide affected soybeans differently. Treatments containing 2,4-D caused less soybean response than the other auxin herbicides. Dicamba at 0.001X reduced soybean yield 19%. Aminopyralid and ACP at 0.01X reduced soybean yields 43% and 16%, respectively. The general ranking of the relative soybean injury caused by the auxin herbicides in this research was 2,4-D < ACP < aminopyralid < dicamba.