Date of Graduation


Document Type


Degree Name

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

Degree Level



Crop, Soil & Environmental Sciences


Tom Barber

Committee Member

Nathan Slatton

Second Committee Member

Jason Norsworthy

Third Committee Member

Edward Gbur


2, 4-D, cotton, Dicamba, Glycine max (L.) Merr, Gossypium hirsutum, soybean


Increases in weed resistance have led to the development of new herbicide-resistant crop technologies, namely the Roundup Ready Xtend® and EnlistTM weed control systems. These new technologies provide resistance to applications of the synthetic auxins dicamba or 2,4-D, respectively, in traited cotton or soybean. It is unknown if there is any resistance to other herbicides within the synthetic auxin group (WSSA Group 4) in these traited crops. Experiments were conducted with auxin herbicides representing all five families within WSSA Group 4. Herbicides were applied either preemergence (PRE) or postemergence (POST) relative to the crop in a single application. Applications were made at a labeled rate (1X) and 1/16 of the labeled rate to LibertyLink, Xtend®, and EnlistTM soybean and cotton cultivars to test for cross- resistance of the herbicides with any of these new technologies. Another study was conducted to determine response (growth and yield) of Roundup Ready and Roundup Ready 2 Xtend® soybean to reduced rates of dicamba [28 g ae ha-1 (1/20x), 7 g ae ha-1 (1/80x), and 3.5 g ae ha-1 (1/160x)] compared with florpyrauxifen-benzyl at rates of 1.46 g ae ha-1 (1/20x), 0.36 g ae ha-1 (1/80x), and 0.18 g ae ha-1 (1/160x). The cross-resistance trial revealed an increased resistance by EnlistTM soybean and cotton, not only to 2,4-D, but also triclopyr and fluroxypyr applied PRE or POST. The Roundup Ready Xtend® crop system was found to possess commercial resistance only to dicamba at either application timing. From this research, it was concluded that resistance of herbicide-resistant crops to a particular herbicide within the WSSA synthetic auxin Group 4 herbicides does not necessarily confer resistance to all herbicides within that group. The drift trial, where low rates of dicamba and florpyrauxifen-benzyl were evaluated following application at the R1 growth stage, indicated that the Roundup Ready 2 Xtend® soybean were resistant to low rates of dicamba, which was expected, but not to low rates of florpyrauxifen-benzyl. The Roundup Ready cultivar was injured by all rates of dicamba and florpyrauxifen-benzyl, with increasing injury as rate of application increased. Yield, the second parameter compared, could not be predicted accurately by visible assessments of injury.