The addition of very low rates of protoporphyrinogen oxidase-inhibiting herbicides to glufosinate does not improve control of glyphosate-resistant horseweed (Erigeron canadensis)

Abstract

Recent research reported synergism between glufosinate plus very low rates of protoporphyrinogen oxidase (PPO)-inhibiting herbicides on select broadleaf weeds. Two field studies, each consisting of four trials, were conducted in 2020 and 2021 in commercial fields with glyphosate-resistant (GR) horseweed in Ontario, Canada. Study 1 evaluated GR horseweed control with glufosinate plus five PPO inhibitors at 5% of the label rate; study 2 evaluated what dose of saflufenacil is needed when co-applied with glufosinate to improve GR horseweed control. In study 1, glufosinate plus very low rates of PPO-inhibiting herbicides provided low GR horseweed control. At site 1, despite the synergistic increase in GR horseweed control with saflufenacil (1.25 g ai ha-1) plus glufosinate (300 g ai ha-1), the level of control did not exceed 42% at 2 and 4 wk after application (WAA); the interaction was additive at 8 WAA. The co-application of glufosinate (300 g ai ha-1) with pyraflufen-ethyl (0.34 g ai ha-1), pyraflufen-ethyl/2,4-D (26.4 g ai ha-1), flumioxazin (5.35 g ai ha-1), fomesafen (12 g ai ha-1), or sulfentrazone (7 g ai ha-1) resulted in an additive interaction for GR horseweed control at 2, 4, and 8 WAA. However, glufosinate plus pyraflufen-ethyl or sulfentrazone was antagonistic at 8 WAA. In study 2, similar doses of saflufenacil were required for 50%, 80%, and 95% GR horseweed control whether glufosinate was included in the mixture or not. Interactions between glufosinate (300 g ai ha-1) plus saflufenacil at 1.56, 3.13, 6.25, and 12.5 g ai ha-1 were antagonistic at 2, 4, and 8 WAA at sites 1, 2, and 3; all other interactions were additive. The results of this research indicate there was little to no benefit of adding very low rates of PPO-inhibiting herbicides to glufosinate to improve GR horseweed control under field conditions.