Response of barnyardgrass (Echinochloa crus-galli) to glyphosate application timing and rate in glyphosate-resistant corn (Zea mays)

Field studies were conducted over 3 yr at two locations to evaluate the effects of glyphosate rate and application timing on barnyardgrass control, seed production, seed viability, and seedbank density the year after herbicide application in glyphosate-resistant corn. Glyphosate was applied at 0, 112, 225, 450, 675, or 900 g ai/ha when barnyardgrass was at the two-, four-, or six-leaf stage of growth. Visual estimates of percent control increased whereas density, dry weight, seed production, and seedbank density the year after treatment decreased as the rate of glyphosate was increased from 0 to 450 g/ha. Increasing the rate of glyphosate from 450 to 900 g/ha (registered rate) had no further effect on any measured parameter. Seed viability was not affected by glyphosate rate nor application timing. Corn yield declined only at a glyphosate rate of 225 g/ha and below. Barnyardgrass control improved as application was delayed to the six-leaf stage because this weed had an extended period of emergence. There was no interaction between glyphosate rate and application timing on any parameter, and yield was not affected by glyphosate-application timing. The use of extremely low glyphosate rates (112 or 225 g/ha) resulted in reduced corn yields, increased barnyardgrass seed production, and seedbank density the year after application.