Late-Season Grass Weed Management with In-Crop and Post-Harvest Herbicides in Twin-Row Glyphosate-Resistant Soybean

Abstract

Emergence of grasses late in the season has become a problem in glyphosate-resistant (GR) soybean production in the southern US. A 3-yr field study was conducted from 2011 to 2013 at Stoneville, MS to determine efficacy of post-harvest and pyroxasulfone-based in-crop herbicides on late-season grasses and yield in twin-row glyphosate-resistant soybean. Experiments were conducted in a split-plot arrangement of treatments in a randomized complete block design with fall herbicides (with and without pendimethalin at 1.12 kg ai ha -1 and paraquatat 0.84 kg ai ha -1 ) as main plots and in-crop herbicides as subplots with four replications. The six in-crop herbicide programs were: glyphosate applied early postemergence (EPOST) at 0.84 kg · aeha -1 followed by (fb) glyphosate late postemergence (LPOST) at 0.84 kg · ha -1 with and without pyroxasulfone preemergence (PRE) applied at 0.18 kg ai ha -1 , pyroxasulfone PRE fb glyphosate at 0.84 kg · ha -1 LPOST or glyphosate at 0.84 kg · ha -1 + S-metolachlor at 1.68 kg ai ha -1 EPOST, pyroxasulfone PRE fb S-meto- lachlor at 1.12 kg · ha -1 + fomesafen at 0.27 kg ai ha -1 EPOST fb clethodim at 0.14 kg ai ha -1 , and a no-herbicide control. Browntop millet, Digitaria spp., and junglerice densities at 2 weeks after LPOST, grass weed dry biomass at harvest, and soybean yield were similar regardless of post- harvest herbicides in all three years. At 2 weeks after LPOST, browntop millet, Digitaria spp. and junglerice densities were greatly reduced in all five in-crop herbicide treatments compared with no herbicide plot in all three years. Grass weed dry biomass in no-herbicide plots was 3346, 6136, and 6916 kg · ha -1 in 2011, 2012, and 2013, respectively and the five herbicide treatments reduced grass weed dry biomass by at least 87%, 84%, and 99% in 2011, 2012, and 2013, respectively. Soybean yield was higher with all five in-crop herbicide treatments compared to no herbicide control in all three years. These results indicate that browntop millet, Digitaria spp., and junglerice infestations can be reduced with pyroxasulfone-based in-crop herbicide programs in twin-row GR soybean.