Degree

Doctor of Philosophy (PhD)

Department

School of Plant, Environmental, and Soil Sciences

Document Type

Dissertation

Abstract

In research conducted using indeterminate soybean [Glycine max (L.) Merr.], fourteen injury criteria observed following dicamba at 0.6 to 280 g ae ha-1 (1/1000 to ½ of 560 g ha-1 use rate) were rated using a scale of 0= no injury, 1= slight, 2= slight to moderate, 3= moderate, 4= moderate to severe, and 5= severe. Greatest crop injury 15 d after treatment (DAT) was observed following dicamba applied at 0.6 to 4.4 g ha-1 at V3/V4 for upper canopy pale leaf margins (3.8 to 4.2) and at R1/R2 for terminal leaf cupping (4.1 to 5.0) and, following 0.6 to 8.8 g ha-1 dicamba applied at V3/V4 for upper canopy leaf cupping (3.8 to 4.8) and upper canopy leaf surface crinkling (3.4 to 4.4). At 15 DAT, injury was no greater than the nontreated control when dicamba rate was as high as 4.4 g ha-1 for lower stem base swelling (V3/V4 application) and for upper canopy leaf rollover/inversion and terminal leaf necrosis (R1/R2 application) and for rates as high as 8.8 g ha-1 for leaf petiole base swelling and stem epinasty (R1/R2 application) and lower stem base lesions/cracking (V3/V4 and R1/R2 applications). In contrast, overall injury ratings (0 to 100%) showed a steady increase as dicamba rate increased. Injury data were analyzed using multiple regression with a forward selection procedure to develop yield prediction models. Variables included in the V3/V4 15 DAT model were lower stem base lesions/cracking, plant height reduction, terminal leaf epinasty, leaf petiole droop, leaf petiole base swelling, and stem epinasty. For the R1/R2 15 DAT model, variables included lower stem base lesions/cracking, terminal leaf chlorosis, leaf petiole base swelling, stem epinasty, terminal leaf necrosis, and terminal leaf cupping. To validate the models, experiments were conducted at two locations and predicted yield reduction for each dicamba rate was compared with observed yield reduction. For dicamba at 0.6 to 4.4 g ha-1, the V3/V4 15 DAT model either underestimated or overestimated observed yield loss by 1 and 3 percentage points and the R1/R2 15 DAT model overestimated observed yield loss by 3 to 5 percentage points.

Date

10-22-2018

Committee Chair

Griffin, James

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