Timing and Application Rate for Sequential Applications of Glufosinate are Critical for Maximizing Control of Annual Weeds in LibertyLink® Soybean

Preserving the utility of glufosinate in both LibertyLink soybean and other glufosinate-resistant crops is critical for managing herbicide-resistant weeds. An experiment with a two-factor factorial arrangement was conducted at the Arkansas Agricultural Research and Extension Center in Fayetteville, AR, in 2015, 2016, and 2017 to evaluate the efficacy of glufosinate in single and sequential applications at various rates on 8–32 cm tall Palmer amaranth, barnyardgrass, and broadleaf signalgrass. Herbicide treatments consisted of glufosinate applied at 454, 595, 738, and 882 g ai ha⁻¹ (Factor 1) with either no sequential application or a sequential application occurring 7, 10, 14, or 21 days after the initial application (DAI) (Factor 2). For treatments that contained a sequential application, the same rate used in the initial application (e.g., 451 g ai ha⁻¹) was also used in the sequential. Regardless of species and rate, sequential applications were always superior to single applications. Palmer amaranth control 3 weeks after the final treatment (WAF) was 8% greater when the sequential application occurred 10 DAI compared to 21 DAI, averaged over glufosinate rates. When at least 595 g ai ha⁻¹ glufosinate was used in a treatment, no differences between the 7-, 10-, 14-, and 21-day sequential application intervals were observed for barnyardgrass or broadleaf signalgrass control, 3 WAF. Soybean yields were greater when the glufosinate applications occurred 7 or 10 d apart compared to 21 d, averaged over glufosinate rates. When large weeds are present in the field, these data suggest that glufosinate should be applied sequentially with a 7- to 14-day interval between applications. If sequential applications of glufosinate are used in combination with a comprehensive weed control management program, the value of the LibertyLink technology should be preserved by mitigating the risk of glufosinate resistance.