Experiments were conducted to determine the efficacy of glyphosate on four common lambsquarters populations collected from Kansas, Nebraska, North Dakota, and Ohio. Glyphosate dose-response studies for common lambsquarters treated at 2.5-, 7.5-, and 15-cm heights showed that glyphosate at 1.1 kg ae ha⁻¹ caused more than 80% injury to 2.5-cm plants but less than 55% injury to 7.5- and 15-cm plants. All populations were susceptible to glyphosate at the 2.5-cm height. The glyphosate rate required to cause 50% injury (GR₅₀) was 430, 500, 500, and 560 g ha⁻¹ for the Kansas, North Dakota, Ohio, and Nebraska populations, respectively. Differential response of common lambsquarters populations was evident with 15-cm plants where the GR₅₀ was glyphosate at 1,010, 1,230, 1,650, and 2,770 g ha⁻¹ for the Kansas, North Dakota, Nebraska, and Ohio populations, respectively. Reduced injury on 15-cm common lambsquarters plants by glyphosate may be partly attributed to reduced glyphosate accumulation per unit of plant tissues and enhanced calcium content in more-developed plants. All four common lambsquarters populations at the early seedling stage were susceptible to glyphosate, but tolerance increased as the plant developed and the extent of tolerance differed among populations.

Nomenclature: Glyphosate common lambsquarters, Chenopodium album L. CHEAL.