The influence of glufosinate in auxinic-based herbicide systems

Palmer amaranth (Amaranthus palmeri S. Watson) is native to the southwestern United States and for decades has been one of the most common weeds in West Texas. In recent years, this weed has been considered one of the most troublesome across the southern United States. The management of Palmer amaranth has changed since the discovery of glyphosate resistant populations in 2005. Dicamba and 2,4-D tolerant cotton (Gossypium hirsutum L.) systems were introduced in 2017 and provide a new opportunity to manage glyphosate resistant populations of Palmer amaranth. The use of glufosinate (Liberty® 280 SL) in auxinic-based herbicide systems may not only improve the management of glyphosate-resistant Palmer amaranth, but also be effective against new developments of herbicide resistance to group 4 modes of action. Four field studies were conducted in a non-crop environment in Lubbock, Texas in 2018 and 2019 to determine the influence of sequential spray order and role of glufosinate when used in a system with dicamba or 2,4-D to control Palmer amaranth at different growth stages. Palmer amaranth control decreased as Palmer amaranth size at initial application increased for both dicamba and 2,4-D studies. A difference in efficacy based on herbicide order was observed for < 10 cm Palmer amaranth in the sequential applications of glufosinate and dicamba with and without acetochlor study. Glufosinate followed by dicamba was less effective than dicamba followed by glufosinate at multiple rating dates in both years. Dicamba + acetochlor followed by glufosinate provided greater Palmer amaranth control than dicamba followed by dicamba at one or more rating dates across all weed sizes. The addition of acetochlor reduced new weed flushes when applied to > 30 cm Palmer amaranth in 6 out of 8 treatments. No treatment controlled > 30 cm Palmer amaranth more than 55%. No consistent difference in efficacy based on herbicide order was observed for > 10 cm Palmer amaranth. Sequential applications of glufosinate or dicamba at 3, 7, and 10 day intervals evaluated the influence of sequential spray timing of dicamba and glufosinate. Sufficient evidence was not obtained in these studies to alter sequential application timing interval recommendation from current label restrictions. Sequential applications of glufosinate and 2,4-D choline evaluated the influence of sequential spray order when applying glufosinate and 2,4-D choline with or without pyrithiobac when controlling 7 to 15 cm and 25 to 30 cm Palmer amaranth. When applied to 7 to 15 cm Palmer amaranth, 2,4-D choline + glyphosate followed by glufosinate controlled Palmer amaranth 100% 10 days after sequential application in 2018 and 97% 21 days after sequential application in 2019. When applied to 25 to 30 cm, Palmer amaranth 2,4-D choline + glyphosate followed by glufosinate controlled Palmer amaranth 98% 11 and 21 days after sequential application in 2018. 2,4-D choline provided greater control when used in the initial application and followed by glufosinate than glufosinate followed by 2,4-D choline in both weed sizes. 2,4-D choline tank-mix applications evaluated the efficacy of tank-mixing 2,4-D choline, 2,4-D choline + glyphosate, and glufosinate when controlling 7 to 15 cm and 25 to 30 cm Palmer amaranth. Seven days after initial application, 2,4-D choline + glufosinate controlled 7 to 15 cm Palmer amaranth 92%, which was greater than 2,4-D choline + glyphosate (88%). Palmer amaranth control ranged from 96 to 99% 7 days after sequential application, but control declined within the following two weeks as a thick, uniform flush of Palmer amaranth emerged and grew quickly, demonstrating the need for residual herbicide in a Palmer amaranth system. Glufosinate served as a complimentary sequential application partner when paired with dicamba or 2,4-D. There was no penalty observed for adding glufosinate to an auxin-based system, and additional modes of action will be more effective to slow the development of weeds resistant to group 4 herbicides when compared to group 4 herbicides used alone.