Survival of horseweed in several glyphosate-tolerant cotton and soybean fields treated with glyphosate at recommended rates preplant and postemergence was observed in Mississippi and Tennessee in 2001 and 2002. Plants originating from seed collected from fields where horseweed escapes occurred in 2002 were grown in the greenhouse to the 5-leaf, 13- to 15-leaf, and 25- to 30-leaf growth stages and treated with the isopropylamine salt of glyphosate at 0, 0.025, 0.05, 0.1, 0.21, 0.42, 0.84, 1.68, 3.36, 6.72, and 13.44 kg ae/ha to determine if resistance to glyphosate existed in any biotype. All biotypes exhibited an 8- to 12-fold level of resistance to glyphosate when compared with a susceptible biotype. One resistant biotype from Mississippi was two- to fourfold more resistant than other resistant biotypes. Growth stage had little effect on level of glyphosate resistance. The glyphosate rate required to reduce biomass of glyphosate-resistant horseweed by 50% (GR50) increased from 0.14 to 2.2 kg/ha as plant size increased from the 5-leaf to 25- to 30-leaf growth stage. The GR50rate for the susceptible biotype increased from 0.02 to 0.2 kg/ha glyphosate. These results demonstrate that the difficult-to-control biotypes were resistant to glyphosate, that resistant biotypes could survive glyphosate rates of up to 6.72 kg/ha, and that plant size affected both resistant and susceptible biotypes in a similar manner.

Weed control is one of the factors that impact alfalfa producers, with negative effects on quality often in the year of establishment. Glyphosate is a broad‐spectrum herbicide that controls many troublesome annual and perennial weeds, and new cultivars that are tolerant of glyphosate application have been developed. The crop response of glyphosate on these new varieties has not been reported. This research examined alfalfa tolerance under field conditions, and high rates were used to challenge the plants to determine the level of safety. Postemergent glyphosate treatments ranging from 0.75 to 3 lb a.e./acre sprayed before each of four alfalfa harvests had no meaningful crop injury in the establishment year or in the subsequent two years. The high glyphosate dose of 9 lb a.e./acre over a 3‐year period caused no reduction in alfalfa yield or nutritive value at any cutting in any of the three years.