Date of Graduation


Document Type


Degree Name

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

Degree Level



Crop, Soil & Environmental Sciences


Nilda R. Burgos

Committee Member

Mary C. Savin

Second Committee Member

Vibha Srivastava

Third Committee Member

Thomas R. Butts

Fourth Committee Member

Matthew Bertucci


Arkansas, palmer amaranth, herbicides, inhibitors, weeds, soil-applied herbicides


Palmer amaranth [Amaranthus palmeri (S.) Wats.] presents both a high genetic diversity and propensity to evolve resistance to herbicides of several sites-of-action which have made it one of the worst weeds in US agriculture. In Arkansas, Palmer amaranth is resistant to herbicides of seven sites-of-action, which are 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibitor, acetolactate synthase inhibitors, microtubule inhibitors, protoporphyrinogen oxidase inhibitors, very long chain fatty acid inhibitors, glutamine synthetase inhibitors, and hydroxyphenylpyruvate dioxygenase inhibitors. Sustainable management requires a better understanding of its biology and that of herbicide environmental fate. This research had five objectives: 1) characterize the current status of Palmer amaranth resistance to S-metolachlor in the Mid-southern US and evaluate alternative control methods for its proactive control, 2) investigate the influence of inter-annual and within-season multiple applications of the herbicide on its dissipation, 3) investigate the dynamics of S-metolachlor dissipation in soil samples collected throughout the growing season, 4) understand the influence of multiple mathematical equations on predictions of dissipation endpoints, and 5) investigate differences in threshold values for the initiation of stomatal closure between herbicide- susceptible and resistant accessions. A general herbicide resistance screening was conducted using seeds collected in Arkansas, Tennessee and Mississippi complemented by dose-response studies of parent populations and F1 progenies. A greenhouse experiment was conducted to evaluate the differences in drought tolerance between S-metolachlor resistant- and susceptible accessions, and between glyphosate- resistant and susceptible plants differing by the number of EPSPS gene copy number. The influence of multiple annual applications on the dissipation half-life of S-metolachlor was evaluated using paired, commercial fields differing by the number of herbicide applications received within the previous six years across five Arkansas counties. The effect of frequency and timing of within-season applications of the herbicide on its dissipation was also evaluated. Five mathematical equations were evaluated for their performance at predicting S-metolachlor dissipation endpoints. Resistance to S-metolachlor was detected in four Arkansas counties and one Mississippi county. Parent populations and F1 lines required up to 7- and 9.2-times more S-metolachlor, respectively, to reduce seedling emergence 50%. The half-life of S-metolachlor was longer for fields with high-use compared to fields with low-use history. A sequential application of S-metolachlor slowed S-metolachlor dissipation compared to the control. S-metolachlor dissipation was faster earlier compared to later during the growing season. Model selection and goodness of fit showed that the biphasic Gustafson and Holden and first-order double-exponential decay models better supported data compared to the single first order (SFO) model. The SFO under-predicted S-metolachlor residues during the later stages of the dissipation process. The threshold value for the initiation of stomatal closure was greater for the S-metolachlor- resistant than the susceptible accession. Glyphosate- resistant and susceptible plants, differing by the number of EPSPS gene copy number, did not show differences in their threshold values for the initiation of stomatal closure. Dissipation is not increased from use, but resistance is occurring and resistance in plant has consequences for plant function under stress conditions. Continued research into plant mechanisms and consequences of increasing resistance needs investigation.