Date of Graduation

5-2018

Document Type

Thesis

Degree Name

Master of Science in Crop, Soil & Environmental Sciences (MS)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor

Tom Barber

Committee Member

Jason Norsworthy

Second Committee Member

Jeremy Ross

Third Committee Member

Nathan Slaton

Fourth Committee Member

Edward Gbur

Keywords

Crop Injury, Dicamba, Drift, Offspring, Soybean, Tank Contamination

Abstract

Introduction of a new dicamba-resistant (Xtend) trait in soybean and cotton will increase dicamba herbicide use. Consequently, concern for injury to sensitive crops from off-target movement and tank contamination will likely increase. For soybean, foliar symptoms associated with dicamba damage do not necessarily reflect yield losses; hence, experiments were conducted to determine the effects of dicamba on soybean growth, yield, and offspring. Low rates of dicamba [1/64X (8.75 g ae ha-1) and 1/256X (2.18 g ae ha-1) of a normal 1X field rate (560 g ae ha-1)] were applied at two vegetative growth stages (V4, V6) and at each reproductive growth stage from R1 to R6. Compared to the nontreated check, dicamba applied during late vegetative and early reproductive growth of soybean caused leaf injury, plant height reduction, and yield loss. Regardless of soybean cultivar, the higher rate of 1/64X resulted in greater yield loss, with R1 being the most sensitive growth stage. Dicamba at 8.75 g ha-1 applied at R1 reduced mature soybean height 35 cm for an indeterminate cultivar and 23 cm for a determinate cultivar. Grain yield was reduced 14% for the indeterminate cultivar and 19% for the determinate cultivar. Injury and height reductions were less apparent when dicamba was applied during later growth stages. Offspring response to dicamba applied to parent plants the previous year was dependent upon application timing and dicamba rate. Negative effects to offspring were observed as reduced seed germination, seedling emergence, plant height, seedling vigor, pod malformation,

and grain yield. Offspring had 17 to 23% injury when parent plants were treated from R4 to R6 with dicamba at 2.18 g ha-1. Seeds from the bottom of the plant were affected more by dicamba than seeds from the top of the plant; however, no relationship existed between grain yield and pod malformation. This research shows that soybean cultivar, growth habit, or planting date may influence soybean recovery from dicamba.

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