Is Horseweed [Conyza Canadensis (L.)] In Oklahoma Becoming Glyphosate-Resistant?.

Due to the continued use of glyphosate in Roundup Ready crops, such as corn, soybean, and cotton, horseweed has become an increasingly troublesome weed in no-till production systems throughout the United States.  However, glyphosate-resistant horseweed has not yet been documented in Oklahoma. To address this concern, horseweed seed samples were collected from randomly selected soybean and fallow fields from north central Oklahoma counties during the fall of 2010. Seed samples were collected from mature horseweed in 15 naturally-occurring populations. A glyphosate-susceptible sample was also collected from one location where glyphosate had not previously been applied. Samples were grown in a greenhouse until the horseweed rosette was 4 to 5 inches in diameter. The plants were sprayed with glyphosate at rates of 0, 0.75, 1.5, or 3.0 lb acid equivalent (ae)/acre of, representing a 0, 1x, 2x, and 4x rate, respectively. Three weeks after treatment, 10 of the 15 horseweed populations were controlled at 50% or less compared to the susceptible population at the standard use rate of 0.75 lb ae/acre.  Results from this trial confirm the presence of glyphosate-resistant horseweed in Oklahoma. In order to successfully control and prevent the further spread of glyphosate-resistant horseweed, farmers must use additional herbicide active ingredients and weed control methods in no-till production systems.