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.
Highlights
The wide-spread adoption of glyphosate resistant crops (GRCs) coupled with over-reliance on glyphosate and inadequate diversity in weed management tactics in GRCs have resulted in weed species shifts [1]-[3]
There were no differences in browntop millet, Digitaria spp. and junglerice density between post-harvest herbicides and no post-harvest herbicide in May 2011, at 1 - 5 days before early postemergence (EPOST) application of in-crop herbicides.Overall, pyroxasulfone applied preemergence gave adequate control of these grass weeds compared to no herbicide in all 3 years
Other researchers have shown that pyroxasulfone applied preemergence effectively controlled (>90%) rigid ryegrass (Lolium rigidum Gaudin) and broadleaf signalgrass [6] [8]
Summary
The wide-spread adoption of glyphosate resistant crops (GRCs) coupled with over-reliance on glyphosate and inadequate diversity in weed management tactics in GRCs have resulted in weed species shifts [1]-[3]. Weed species shifts refers to a relative change in weed population (abundance) or species (diversity) along with lateseason weed emergence in an agricultural system in response to weed management tactics [2]. Weed species shifts in GRCs are a result of weeds that have escaped control because of a natural high level of tolerance to glyphosate or glyphosate avoidance from late-emerging cohorts. Emergence of weeds late in the cropping season has become a problem in GRCs [2]. The long time period between harvest and killing frost (October-November) provides a favorable environment for certain weeds to emerge and complete a life cycle [4]
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