Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Plants, Soils, and Insects

Major Professor

Thomas C. Mueller

Committee Members

Robert M. Hayes, Vincent R. Pantalone, C. Neal Stewart, John B. Wilkerson


This research had multiple objectives: 1) confirm resistance to glyphosate in a regional horseweed [Conyza canadensis (L.) Cronq.] population; 2) determine the location of glyphosate-resistant horseweed in the lower mid-west and mid-south states; 3) determine the level of glyphosate-resistance in horseweed biotypes from Tennessee; 4) examine the accumulation of shikimate in both glyphosate-resistant (GR) and glyphosate-susceptible (GS) horseweed plants.

The analytical procedure to determine shikimate used extraction with 1 M HCl for 16 hr followed by liquid chromatography using photo-diode array detection, and shikimate recoveries were ≥ 82%. Glyphosate application of either 0.84 kg ae/ha (the standard application rate) and 3.8 kg ae/ha to susceptible plants caused complete plant death to susceptible plants. Glyphosate applied at 3.8 kg ae/ha to putative resistant populations caused < 15% growth reduction as determined by visual evaluations, and fresh weights of these resistant plants 17 days after glyphosate treatment (DAT) were reduced an average of 45% in one population (susceptible), and were not affected in a different population (denoted resistant). This direct comparison conclusively confirms that horseweed plants collected in western Tennessee in 2002 were resistant to four times the normal application dosage of glyphosate. The GR horseweed biotypes still exhibited some herbicidal effects from glyphosate, such as yellowing in the most actively growing, apical shoot meristems. The yellowing in the shoot apices was transitory, older leaf tissue became necrotic, and the plants recovered from this damage to continue growth. Shikimate concentrations in all untreated horseweed plants were less than 100 μg/g, which was significantly less than all plants which had been treated with 0.84 kg ae/ha of glyphosate. Unexpectedly, shikimate accumulated (> 1000 μg/g) in both resistant populations and in the GS population. However, there were differences in shikimate accumulation patters between resistant and susceptible horseweed biotypes. Shikimate concentrations in resistant populations declined about 40% from 2 to 4 DAT, while shikimate concentrations in susceptible horseweed plants increased about 35% from 2 to 4 DAT. The confirmed resistance of a widespread weed implies that alternative control strategies for GR horseweed will be needed in those no tillage production systems were it commonly occurs.

Horseweed seed were collected in Illinois, Indiana, Kentucky, Mississippi, Missouri and Ohio to determine susceptibility of different horseweed biotypes to glyphosate. Horseweed resistant to glyphosate were found in Mississippi, Ohio, and western Tennessee. In a separate experiment examining Tennessee biotypes, a dose response curve demonstrated that four times as much glyphosate was needed to achieve a 50% fresh weight reduction (GR50) in resistant biotypes when compared to a susceptible biotype. Resistant biotypes from Tennessee displayed a GR50 of 1.6 kg/ha, as compared to a GR50 of 0.4 kg/ha in a susceptible horseweed population.

A more comprehensive analysis of the response of shikimic acid levels in shoot and root tissue of GS and GR horseweed biotypes was conducted. Both horseweed biotypes displayed an increase in shikimic acid indicating that 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) remained sensitive to glyphosate. Shikimic acid levels in both shoots and roots of GS horseweed displayed a increasing sigmoidal response to glyphosate, while in GR horseweed shikimic acid levels displayed an increasing log normal peak response with a maximum concentration occurring around 72 hours after treatment (HAT) in both shoot and root tissue. Shikimic acid concentration in GR horseweed began to decrease between 72 and 96 HAT indicating that the shikimic acid pathway resumed at least partial function in the presence of glyphosate. At 168 HAT shikimic acid levels in GS horseweed shoot tissue displayed a 6:1 increase and a 3:1 increase in root tissue when compared to GR horseweed. This ratio corresponds to previously observed differences in whole plant sensitivity to glyphosate for GS and GR horseweed.

These results imply that horseweed resistance to glyphosate is not due to a change in the site of herbicide action. The mechanism of resistance appears to be similar to GR ryegrass and different from glyphosate tolerant crops. GR horseweed biotypes required four times more glyphosate to achieve 50% growth reduction when compared to GS horseweed biotypes. Shikimic acid decrease over time could be due to the presence of three isoforms of EPSPS and possible glyphosate induced amplification of the genes coding for EPSPS. Changes in EPSPS may allow the shikimate acid pathway to operate depleting the shikimic acid pool, leading to continued plant growth.

Files over 3MB may be slow to open. For best results, right-click and select "save as..."