Date of Graduation

5-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Crop, Soil & Environmental Sciences (PhD)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor/Mentor

Norsworthy, Jason K.

Committee Member

Barber, L. Tom

Second Committee Member

Roberts, Trenton L.

Third Committee Member

Mauromoustakos, Andronikos T.

Fourth Committee Member

Rupe, John C.

Keywords

Dicamba volatility; Herbicide residue; Off-target movement

Abstract

The expansion of dicamba-resistant technology increased the use of dicamba in season and relocated the off-target movement towards the growing season, negatively affecting susceptible vegetation, such as soybean. This research aimed to understand different aspects of the off-target movement of dicamba and impact on susceptible soybean. A study determined the accumulation of dicamba in soybean seed (grain) when parental plants at the pod-filling stage were treated with 1/200 of a labeled rate of dicamba at 560 g ae ha-1 and radiolabeled herbicide used as a tracer. This research found that 44% of the total herbicide absorbed by the parent plants was transported toward seeds, and at least 99% of the total remained active, potentially impacting seed quality. Off-target movement investigations conducted by state regulatory officials include collecting and analyzing plant tissue for pesticide content and photos of the alleged damage. The second experiment assessed the persistence of dicamba and 2,4-D in Palmer amaranth and 2,4-D- and dicamba-resistant soybean. The findings revealed that the likelihood of detecting either herbicide in plant samples decreased rapidly following exposure; therefore, early sampling is critical to recovering potential herbicides in plant tissue. A third experiment evaluated the influence of volatilization and suspension of physical particles on the off-target movement of dicamba plus glyphosate with imazethapyr, a non-volatile herbicide added as a tracer. Results showed that the ratio of dicamba to imazethapyr detected in air samples was at least 50:1, several orders of magnitude more than the ratio of herbicide applied to the field (5.3:1), indicating that volatility contributed to most of the off-target movement that occurred. The fourth study established relationships between soybean responses, including visible injury and height reduction and concentration of volatilized dicamba. A predicted dicamba concentration of 1.60 ng m-3 per day resulted in 10% visible soybean injury The fifth study investigated differences in the off-target movement of dicamba impacted by herbicide and adjuvant mixtures or types of surfaces treated, using low tunnel trials. These experiments focused on causes of enhanced dicamba volatility where it was found that both glufosinate and glyphosate increase detection of the dicamba in air. Dicamba volatility was reduced by 70% when a volatility reduction agent was added to dicamba treatments, an adjuvant currently required for any in-crop application with the herbicide.

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