Crop Response to Low-Dose Dicamba

The introduction of dicamba-resistant (DR) soybean [Glycine max (L.) Merr.] and cotton (Gossypium hirsutum L.) followed by a label allowing over-the-top applications of dicamba in the 2017 provided growers an additional option for broadleaf weed control. Because non-DR soybean is sensitive to low concentrations of dicamba, postemergence dicamba applications present growers choosing to plant non-DR soybean with concerns of damage from off-target movement through physical drift and volatilization of the herbicide as well as tank-contamination from prior dicamba application. Consequences of low-dose dicamba exposure on sensitive vegetative and reproductive soybean have been well researched, but little is known regarding the interaction between low-dose dicamba exposure and commonly applied contact herbicides. There is also limited research assessing repercussions caused by dicamba exposure during reproductive development in corn (Zea mays L.) and rice (Oryza sativa L.), two common cereal crops in Arkansas in proximity to soybean and cotton. Experiments included evaluating the potential for dicamba to elicit a hormetic response to sensitive soybean, determining if contact herbicides exacerbated off-target dicamba symptomology, and evaluating potential for low-dose dicamba exposure on reproductive corn and rice to reduce grain yield. Low doses of dicamba did not improve soybean grain yield, which indicates a hormetic response is unlikely. Contact herbicides such as glufosinate and acifluorfen applied in close proximity to a low dose of dicamba increased visible auxin symptomology to soybean at 21 and 28 days after treatment (DAT) compared to dicamba alone, but no effect on grain yield occurred. Soybean was injured more when dicamba exposure followed a glufosinate application than when dicamba preceded glufosinate or was applied in a mixture with glufosinate, with yield reductions resulting when the contact herbicide was applied 7 to 10 days after a V3 dicamba exposure. Visible injury to corn from dicamba was negligible (0%), but rice treated with dicamba at 56 and 560 g ae ha-1 demonstrated auxin symptomology which led to reductions in grain yield. Corn grain yield could only be reduced from dicamba at 560 g ae ha-1. In conclusion, dicamba tank-contamination and possibly off-target movement pose a greater threat to sensitive soybean regarding visible symptomology and yield as opposed to corn and rice, and it is unlikely that soybean exposure to low-dose dicamba will improve grain yields