Location: Crop Production Systems Research Unit
Title: Saflufenacil efficacy on horseweed and interaction with glyphosate Authors
Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 11, 2013
Publication Date: N/A
Interpretive Summary: The rapid and widespread adoption of glyphosate-resistant crops, associated with the intense use of glyphosate and lack of rotation with non-glyphosate-resistant crops has resulted in evolution of glyphosate-resistant weeds including horseweed in Mississippi and several US states. Glyphosate-resistant horseweed could negatively impact row crops such as corn, cotton, rice, and soybean. Scientists from the Crop Production Systems Research Unit (USDA-ARS), Mississippi State University, and Pioneer Hi-Bred International, Inc. conducted studies to determine the most efficacious adjuvant system for the control of horseweed with saflufenacil, a new herbicide alternative for horseweed control, to investigate interactions between saflufenacil and glyphosate mixtures on the control of horseweed, and to determine patterns of uptake and translocation of glyphosate applied alone and in combination with saflufenacil in horseweed. Results from field studies indicated that methylated seed oil at 2%, volume basis, was the best adjuvant system for enhancing saflufenacil activity on horseweed. Greenhouse studies indicated that saflufenacil was highly efficacious (=93% control) on both glyphosate-resistant and –susceptible populations and its interaction with glyphosate was additive in nature. The addition of saflufenacil reduced glyphosate translocation in horseweed populations, but did not reduce control due to the highly phytotoxic nature of the saflufenacil molecule itself.
Technical Abstract: Field studies were conducted in 2008 and 2009 to evaluate the effect of adjuvants on the control of horseweed with saflufenacil at 0.025 kg ai ha-1. Addition of 1 and 2% methylated seed oil (MSO) to saflufenacil controlled 91 and 93% of horseweed, respectively at 14 days after treatment (DAT). Horseweed control with the addition of nonionic surfactant (NIS) (78%) was no better than saflufenacil alone (78%) 14 DAT. The addition of 1 and 2% crop oil concentrate (COC) to saflufenacil improved control of horseweed to 85 and 86%, respectively, over the addition of NIS, but was not comparable to either rate of MSO 14 DAT. Greenhouse studies were conducted in 2009 to evaluate the addition of glyphosate (0.4 to 1.68 kg ae ha-1) to saflufenacil (0.012 to 0.050 kg ha-1) on the control of previously characterized glyphosate-resistant (GR) and glyphosate-susceptible (GS) horseweed. Saflufenacil at all rates controlled both GS and GR populations at least 93% and 100% at 14 and 21 DAT, respectively and control of horseweed with the combination of saflufenacil + glyphosate was additive. Studies were conducted in 2009 to determine saflufenacil effects on absorption and translocation of glyphosate in the GR and GS horseweed populations. Overall, GS horseweed absorbed 12, 13, and 6% more 14C-glyphosate than GR horseweed at 24, 48, and 72 h after treatment (HAT), respectively. The addition of saflufenacil reduced 14C-glyphosate translocation in GR and GS horseweed at least 6%; however, due to the exceptional efficacy of saflufenacil on horseweed these reductions did not reduce control.