Response of common lambsquarters (Chenopodium album) to glyphosate application timing and rate in glyphosate-resistant corn

Field studies were conducted over 3 yr at two locations to evaluate the effect of glyphosate rate and time of application on common lambsquarters control, density, dry weight, seed production, and the number of seedlings emerging from soil cores taken the year after herbicide application in glyphosate-resistant corn. Glyphosate was applied at 0, 112, 225, 450, 675, or 900 g ai/ha when common lambsquarters were at the two-, four-, or six-leaf stage of growth. Nicosulfuron was applied to all experimental areas to control annual grasses. Visual estimates of percent control increased, whereas density, dry weight, seed production, and seedlings emerging the year after treatment decreased as the rate of glyphosate was increased from 0 to 450 g/ha. Increasing the glyphosate rate above 450 g/ha had little effect on these parameters. Corn yield declined only at glyphosate rates below 450 g/ha. Time of application had no effect on common lambsquarters control and corn yield because little emergence occurred after the first glyphosate application. There was no interaction between glyphosate rate and time of application for any of the parameters evaluated. In these studies, the application of glyphosate at half the manufacturer's registered rate provided control of common lambsquarters equivalent to the full-registered rate with no measured increase in weed seed production and no increase in weed seedlings emerging from soil cores the year after herbicide application. The results suggest that in some cases the use of reduced herbicide rates can provide excellent weed control and maintain crop yields, while reducing the cost of production and the environmental impact of herbicides. The use of extremely low rates (112 or 225 g/ha), however, resulted in reduced corn yields, increased common lambsquarters seed production and seedlings emerging the year after application, and possibly increased weed management costs in subsequent years.