Control of common lambsquarters with POST applications of glyphosate in glyphosate-resistant crops generally has been effective. In 2002, common lambsquarters plants from Westmoreland County, VA, were identified after not being controlled with a POST glyphosate application in glyphosate-resistant soybean. Plants from this site that survived glyphosate were collected in both 2002 and 2004. The objective of this research was to evaluate the susceptibility of F1 and F2 progeny from these common lambsquarters plants, relative to the susceptibility of common lambsquarters collected in Montgomery County, VA. F1 progeny of the Westmoreland plants from both the 2002 and 2004 collections showed reduced response to glyphosate relative to the Montgomery collection. Vigor reduction of F1 progeny from three 2004 Westmoreland source plants with 0.84 kg ae ha⁻¹ of glyphosate ranged from 66 to 85% at 28 d after treatment (DAT), compared with 89% for the Montgomery collection. Evaluation of four Westmoreland F2 common lambsquarters lines derived from 2002 collections indicated significant differences in glyphosate sensitivity. Fifteen F2 lines were generated from 2004 collections from each of three Westmoreland source plants and from the Montgomery source. For the least sensitive Westmoreland source, vigor reduction ranged from only 24 to 36% across F2 lines in response to 1.68 kg ha⁻¹ of glyphosate at 28 DAT relative to 55 to 100% for the Montgomery source. I₅₀ estimates for fresh weight reduction were 0.91 and 0.32 kg ha⁻¹, for these sources, respectively. Sequential treatments of 0.42, 1.26, and 1.68 kg ha⁻¹ applied at 3-wk intervals to the least susceptible 2004 Westmoreland F2 line resulted in only 37% vigor reduction and no mortality among 360 treated plants.

Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L. CHEAL; soybean, Glycine max L. Merr