Defining Glyphosate and Dicamba Drift Injury to Field Pea, Dry Bean, and Potato

Herbicide spray drift is the most common complaint in relation to pesticide use in North Dakota. With the development of glyphosate-resistant crops and the quick conversion to these cropping systems, glyphosate was often the herbicide suspected for off-target injury. However, dicamba-resistant soybean and the adoption of this technology to combat glyphosate-resistant weed problems, may cause even more drift injury to off-target horticultural crops. Dicamba is known to be volatile and can remain in spray equipment if not cleaned properly, which may injure off-target plants during spraying operations. An overview of six simulated drift studies using glyphosate, dicamba, and mixtures of both herbicides on field pea (Pisum sativum subsp. arvense L.), dry bean (Phaseolus vulgaris L.), and potato (Solanum tuberosum L.) will be presented. All studies used three sub-lethal doses at 10-fold increments of glyphosate and dicamba, along with high, medium, and low doses of both herbicides mixed together. The highest dose for each herbicide did vary for the three crops due to sensitivity differences. Herbicide doses were targeted for the R1 stage with field pea and dry bean, and at tuber initiation for potato. Visual injury observations were made 10 and 20 days after treatment (DAT), while yields and grades were collected at the end of the growing season. For field pea, visual injury was relatively low ≤ 21% at 10 DAT and decreased by 20 DAT. Visual injury symptoms were greater for dry bean, especially when doses included dicamba. For potato, visual injury was greatest for doses that included dicamba, but were relatively low ≤ 13% at 10 DAT and increased two-fold or more by 20 DAT. Yield reduction compared to the untreated was greatest when doses included dicamba, regardless of the crop. Results suggest that drift injury potential to field pea, dry bean, and potato will be greater if a dicamba-resistant soybean crop is adjacent and upwind compared to a glyphosate-resistant crop.