Sensitivity of rice to low rates of glyphosate and glyphosate plus dicamba at multiple growth stages

Off-target movement of glyphosate can be severely injurious to rice (Oryza sativa L.). Glyphosate is routinely mixed with dicamba and applied to crops with resistance to both herbicides, such as glyphosate- and dicamba-resistant corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] cultivars. This research was conducted to evaluate the effects of low rates of glyphosate and glyphosate plus dicamba on rice at multiple application timings. Separate field experiments were conducted and were repeated in 2018 and 2019 near Stuttgart, AR. The first experiment evaluated rice's response to glyphosate alone; a second experiment evaluated responses to glyphosate applied as a mixture with dicamba over several rice growth stages. The rates of glyphosate alone evaluated were 0.023, 0.047, 0.187, and 0.375 oz acid equivalent (ae) acre(-1). Mixtures of glyphosate plus dicamba were evaluated at 0.050 plus 0.025, 0.2 plus 0.1, and 0.8 plus 0.4 oz ae acre(-1), respectively. Visible injury to rice from glyphosate alone was never more than 3%, averaged over application timings. Unique symptomology and greater yield losses were observed for glyphosate mixed with dicamba. There was no reduction in rough rice grain yield after treatment with glyphosate alone. For the glyphosate plus dicamba experiment, no more than 13% injury occurred from any treatment. Rough rice grain yield was reduced by 21% relative to the nontreated control for the highest rate of glyphosate plus dicamba, averaged over application timings. Care should be taken to prevent off-target movement of mixtures of glyphosate plus dicamba to neighboring rice.