It’s hard to rely solely on post-emergence weed control, says Iowa State University Extension Weed Management Specialist Bob Hartzler. “Fields with heavy infestations should be treated as quickly as possible after weed emergence,” he says.

A multi-state project evaluated the effect of time of weed removal in glyphosate-resistant corn. In the 35 experiments, the average yield loss was 2% when the initial glyphosate application was made to 2.5-in. weeds (see table below). Delaying the application until weeds were 5 in. tall doubled the yield loss. The variability in the critical period (the period before weeds impact yields) can be seen looking at the yield response at the sites with the least and most competitive environments. At sites with low levels of competition, corn yield loss was not affected when application was delayed until weeds were 7.5 in. At the other end of the spectrum, corn yields were reduced 13% when weeds were only 2.5 in. at locations with high weed competition.

Effect of early-season competition on corn yields
(Gower et al. 2003. Weed Technol.)

For details see http://www.weeds.iastate.edu/mgmt/2008/timeliness.htm.