Two rapid, nondestructive assays were developed and tested for their potential in differentiating glyphosate-resistant from glyphosate-susceptible biotypes of horseweed. In one assay, leaves of glyphosate-resistant and -susceptible corn, cotton, and soybean plants, as well as glyphosate-resistant and -susceptible horseweed plants, were dipped in solutions of 0, 300, 600, and 1,200 mg ae L⁻¹ glyphosate for 3 d, and subsequent injury was evaluated. In the second assay, plant sensitivity to glyphosate was evaluated in vivo by incubating excised leaf disc tissue from the same plants used in the first assay in 0.7, 1.3, 2.6, 5.3, 10.6, 21.1, 42.3, and 84.5 mg ae L⁻¹ glyphosate solutions for 16 h and measuring shikimate levels with a spectrophotometer. The leaf dip assay differentiated between glyphosate-resistant and -susceptible crops and horseweed biotypes. The 600 mg L⁻¹ rate of glyphosate was more consistent in differentiating resistant and susceptible plants compared with the 300 and 1,200 mg L⁻¹ rates. The in vivo assay detected significant differences between susceptible and glyphosate-resistant plants of all species. Shikimate accumulated in a glyphosate dose–dependent manner in leaf discs from susceptible crops, but shikimate did not accumulate in leaf discs from resistant crops, and levels were similar to nontreated leaf discs. Shikimate accumulated at high (≥ 21.1 mg ae L⁻¹) concentrations of glyphosate in leaf discs from all horseweed biotypes. Shikimate accumulated at low glyphosate concentrations (≤ 10.6 mg L⁻¹) in leaf discs from susceptible horseweed biotypes but not in resistant biotypes. Both assays were able to differentiate resistant from susceptible biotypes of horseweed and could have utility for screening other weed populations for resistance to glyphosate.

Nomenclature: Glyphosate; horseweed, Conyza canadensis (L.) Cronq. ERICA; corn, Zea mays L. ‘Dekalb 687RR’, ‘Pioneer 31B13’; cotton, Gossypium hirsutum L. ‘Delta and Pine Land 444RR’, ‘Suregrow 747’; soybean, Glycine max (L.) Merr. ‘Delta and Pine Land 4748’, ‘Asgrow 4702RR’.