ABSTRACT

In Bossier parish in northwest Louisiana, suspected resistance of cocklebur to MSMA was reported in 1991. Greenhouse and field studies were initiated in 1992 to evaluate the susceptibility of cocklebur at this site to MSMA and Staple (DPX-PE350). In a greenhouse study, cocklebur seed collected from plants that had survived MSMA treatment (resistant cocklebur) along with seed collected in West Feliciana parish where MSMA had never been used (susceptible cocklebur) were treated with MSMA at 2, 4, and 6 lb a.i./A. Plants were 3 to 5 inches in height at treatment time and an untreated check was included for comparison. At 2 wk after treatment, susceptible cocklebur was completely controlled with MSMA at 2 lb/A, but control of resistant cocklebur at 6 lb/A was only 14%. At 4 wk after treatment, fresh weed biomass was similar for both susceptible and resistant cocklebur when MSMA was not applied. No appreciable biomass of the susceptible cocklebur was measured following MSMA application. Biomass of resistant cocklebur, however, for the untreated check and for MSMA applied at 6 lb/A was equivalent. In a field study conducted at the site where cocklebur seed were collected in Bossier parish, no more than 13% cocklebur control was obtained with 4 lb/A MSMA applied to 2 to 3 inch weeds. Application of Staple at the same time at 1 oz a.i./A controlled 95% cocklebur. Application of Staple at the same rate 1 wk later when cocklebur was 5 to 6 inches tall provided 91 % control. Results of these studies indicate that cocklebur at the Bossier parish site were resistant to MSMA, but were controlled effectively in the field study with Staple.