Journal of Experimental Agriculture International

Aims: Determine the influence of sequential spray order and role of glufosinate when used in a system with dicamba to control Palmer amaranth at three different growth stages.

Study design:  Randomized complete block design with four replications

Place and duration of Study: A fallow, non-crop field at the Texas A&M AgriLife Research and Extension Center, Lubbock, Texas, during the 2018-2019 growing seasons

Methodology: Herbicides were applied to < 10 cm, 10 to 20 cm, and > 30 cm Palmer amaranth with a handheld 1.93m CO₂-pressurized backpack calibrated to deliver 140 L ha^-1 at 207 kPa. Palmer amaranth control was visually estimated on a scale of 0 (no control) to 100% (complete plant death) relative to the nontreated control. Palmer amaranth biomass and density were taken when all plots reached 50% or less control in 2019. Palmer amaranth control, biomass, and density were subjected to analysis of variance and means were separated using Fisher’s Protected LSD at an alpha of 0.05.

Results: Palmer amaranth control decreased as Palmer amaranth size at initial application increased. A difference in efficacy based on herbicide order was observed for < 10 cm Palmer amaranth. Glufosinate followed by dicamba was less effective (76-83%) than dicamba followed by glufosinate (93-96%) at 2 of 4 rating dates following sequential applications 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.

Conclusion: Glufosinate served as a complimentary partner in the dicamba-based system, and additional modes of action will be more effective to slow the development of resistance to group 4 herbicides when compared to repeated use of a group 4 herbicide used alone.