Theses and Dissertations

Issuing Body

Mississippi State University


Shaw, R. David

Committee Member

Koger, H. Clifford

Committee Member

Bond, A. Jason

Committee Member

Massey, H. Joseph

Committee Member

Reddy, N. Krishna

Other Advisors or Committee Members

Wixson, B. Marshall

Date of Degree


Document Type

Dissertation - Open Access


Weed Science

Degree Name

Doctor of Philosophy


College of Agriculture and Life Sciences


Department of Plant and Soil Sciences


Interest and research on herbicide-resistant cropping systems has increased dramatically since the introduction of glyphosate-resistant crops in 1996. New advances in herbicide-resistant cropping systems, such as dicambaresistant soybean and cotton, provide opportunities to help alleviate selection pressure currently applied by glyphosate-only systems. While there is no doubt dicamba-resistant genetics will have a huge impact on production practice, there are questions that must be answered about possible interactions with dicamba and glyphosate tank mixtures. The primary objectives of this research were to evaluate the effect of glyphosate/dicamba combinations on common Mississippi weed species, as well as determine effects of these combinations on absorption and translocation of dicamba. Four monocots: johnsongrass, barnyardgrass, large crabgrass, and broadleaf signalgrass; and four dicots: sicklepod, hemp sesbania, prickly sida, and pitted morningglory, were chosen to represent troublesome weed species. Plants were sprayed at the 4±1 leaf stage with glyphosate, dicamba, and combinations of the two herbicides. Rates were chosen with the goal of achieving 40 to 70% control in order to determine synergistic/antagonistic responses. Antagonism was observed in each species tested. Increasing rates of both herbicides alleviated the antagonism in most weeds. A synergistic response was observed in all graminaceous species and pitted morningglory when herbicide rates increased. Barnyardgrass and sicklepod were selected to quantify absorption and translocation of 14C-dicamba in order to account for interactions observed from tank-mix combinations. Rates for dicamba and glyphosate were selected based on results from the interaction study. Dicamba, glyphosate, and tank-mix combinations were applied to sicklepod and barnyardgrass before treatment with 14C-dicamba. Plants were harvested 4, 12, 24, and 72 h after treatment. The addition of glyphosate to dicamba resulted in reduced translocation of 14C-dicamba in both species. While the data did indicate a translocation interaction, glyphosate and dicamba combinations effectively overcame antagonism effects when higher rates were applied on sicklepod and barnyardgrass.