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Herbicide Interactions between Glufosinate and Three Fomesafen-Containing Herbicide Products as Affected by Weed Size and Spray Droplet Size

Chris J Meyer, Jason K Norsworthy and Greg R Kruger

Two effective herbicides with different sites of action (SOA) are recommended for control of problematic weeds such as Palmer amaranth. When a LibertyLink^® soybean variety is planted in the Midsouth, USA, glufosinate is often mixed with fomesafen to control Palmer amaranth and other common weed species. However, mixtures of glufosinate and fomesafen could be antagonistic, specifically when applied to grass species. A two-factor factorial experiment (herbicide treatment by weed size) was conducted at the Northeast Research and Extension Center in Keiser, Arkansas, to evaluate mixtures of glufosinate and three fomesafen-containing products for weed control and herbicide antagonism. Twelve herbicide treatments consisting of glufosinate and fomesafen herbicides alone, and in various combinations, were applied at two weed sizes (10 and 30 cm). Mixtures of glufosinate plus a fomesafen product, regardless of weed size, resulted in ≥ 96% control of PPO-inhibitor-susceptible Palmer amaranth. The addition of a fomesafen product to glufosinate had a negligible effect on control of barnyardgrass and tended to improve control of large crabgrass when compared to glufosinate alone (30 cm weed size). Most of the interactions between glufosinate and fomesafen were additive and it did not appear that one herbicide was negatively affecting the activity of the other. Generally, a premix of fomesafen+S-metolachlor provided better control than either Reflex or Flexstar herbicides alone and mixed with glufosinate. The improved control from the premix of fomesafen+Smetolachlor may partially be explained by droplet size; the premix produced smaller D_v50 (245 μm), compared to 289 and 303 μm, for Flexstar and Reflex herbicides, respectively. Smaller droplet sizes typically improve efficacy of contact herbicides such as glufosinate and fomesafen. To maximize efficacy of glufosinate plus fomesafen mixtures, use a premix of fomesafen+S-metolachlor, which also has the added benefit of multiple herbicide SOA for residual weed control.