Date of Graduation


Document Type


Degree Name

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

Degree Level



Crop, Soil & Environmental Sciences


Jason K. Norsworthy

Committee Member

Nilda R. Burgos

Second Committee Member

Ken L. Smith

Third Committee Member

Terry W. Griffin

Fourth Committee Member

Edward E. Gbur

Fifth Committee Member

Ken Korth

Sixth Committee Member

Leo Espinoza


Biological sciences, Amaranthus palmeri, Glyphosate resistance, Herbicide resistance, Palmer amaranth


Since 2000, there have been thirteen weed species confirmed resistant (R) to glyphosate in the United States, six of those resistant species are present in Arkansas. The goal of this research was to confirm and to determine the level of resistance in two R Palmer amaranth biotypes from Mississippi (MC-R) and Lincoln (LC-R) Counties, Arkansas, and one susceptible (S) biotype from Clarendon County, South Carolina, which had never been exposed to glyphosate. Shikimic acid concentration over time was significantly greater in the S biotype than both the MC-R and LC-R biotypes. The lethal dose required to kill 50% (LD50) of the population was 2,255 and 3,223 g ae ha-1 for the MC-R and LC-R biotypes, respectively, and it was hypothesized that the two Arkansas biotypes each had a different resistance mechanism. Results indicate metabolism of glyphosate to its major metabolite, aminomethylphosphonic acid (AMPA), was not responsible for resistance in any biotype. Reduced absorption in the LC-R and limited translocation from the treated leaf in the MC-R were at least partially responsible for the observed resistance to glyphosate. The LC-R biotype effectively colonized a field within two years of a single resistant female producing ~20,000 seed. Cotton lint yield was reduced over 100 kg ha-1 by some densities of LC-R Palmer, depending on the soil and relative elevation in that region. Several resistant management options exist in cotton; however, results indicate that timely herbicide applications based off of Palmer amaranth size are required for effective season-long control and management of the soil seedbank.