Glyphosate-resistant (GR) weeds, including giant ragweed, are among the most challenging weeds for growers to control in cotton. A field study was conducted in 2011 and 2012 to determine the competitiveness of giant ragweed with densities of 0, 0.1, 0.2, 0.4, 0.8, or 1.6 plants m⁻¹ of row. Early in the growing season, giant ragweed competition with densities of at least 0.8 plants m⁻¹ row reduced cotton height compared with the weed-free control. Based on node above white flower (NAWF) and node above cracked boll (NACB) data, a delay in cotton maturity was observed for treatments with giant ragweed present at a density of 1.6 m⁻¹ of cotton row for NAWF and 0.8 m⁻¹ or 1.6 m⁻¹ of row for NACB. Lint yield losses of 50% were estimated for cotton with rows growing along side of giant ragweed at a density of 0.26 plants m⁻¹ row. Cotton in rows located 140 cm away from giant ragweed required an estimated 1.85 plants m⁻¹ row to reduce yield by 50%. These data suggest that giant ragweed sphere of influence was at least 1 m wide. Cotton fiber quality was not affected by giant ragweed at any density. Giant ragweed is a highly competitive weed in cotton, even at low densities, and efforts should be implemented to control giant ragweed early in the season to prevent cotton yield loss.

Nomenclature: Cotton, Gossypium hirsutum L., giant ragweed, Ambrosia trifida L., glyphosate resistant, GR.