Field research was conducted in 1999 and 2000 to determine the effect of reduced glyphosate rates on growth and yield of nonglyphosate-resistant cotton. Rates of 9, 18, 35, 70, 140, and 280 g ha⁻¹, representing 0.008, 0.016, 0.031 0.063, 0.125, and 0.25, respectively, of the maximum use rate per application (1,120 g ha⁻¹), were applied to cotton at the two-, five-, or nine-node growth stage. On the basis of visual injury estimates, cotton was more tolerant to glyphosate at the nine-node than at earlier growth stages. Plant dry weight was reduced with 70 g ha⁻¹ of glyphosate or higher, when applied at the two- and five-node growth stages in two of three experiments. Dry weight was not affected by glyphosate at the nine-node stage. Plant height also was unaffected by glyphosate rates below 70 g ha⁻¹, but height reduction was noted for all growth stages by experiment combinations, with the exception of the nine-node application for both experiments in 2000, with herbicide rates of 70 g ha⁻¹ or higher. Cotton maturity delay, as noted by an increase in node above white flower number, was observed only at the highest glyphosate rate applied to two- and five-node cotton in one of three experiments. Percent open boll data analysis indicated a decreased opportunity of observing an open boll with increasing glyphosate rate, and this effect was greater at the five-node compared with the two- and nine-node stages in two of three experiments. Seedcotton yield after all glyphosate applications was equivalent to that for the nontreated control.

Nomenclature: Glyphosate; cotton, Gossypium hirsutum L. ‘Stoneville 474’, ‘DP33B’.