To harvest the highest possible corn yields with conservation tillage, weed scientists maintain early weed control is essential. Weeds that emerge at the same time as corn plants are much more competitive with a growing crop than later-emerging weeds.

“This is the number one driver in weed control,” says C.J. Swanton, a weed scientist at the University of Guelph in Guelph, Ontario. “The time of weed emergence, relative to the crop, is the most important driver and knowing when different weeds emerge is critical. The critical period is the window in crop development when weeds must be controlled to prevent unacceptable yield losses. Clean fields at the end of the growing season mean nothing when it comes to yield.”

Swanton says the critical period for weed control in no-tilled corn is between the third and eighth leaf stage. “While this is very broad and varies between fields,’ he says, “this is the time when you really need effective weed control.”

His studies with early-season weed control reveal yield losses ranging from 2 to 33 bushels per acre for each day that herbicide spraying is delayed. “You can’t afford to wait to spray until the weeds come up,” he says. “The yield loss occurs quickly and is lost forever with both corn and soybeans.”

Swanton says annual weeds that emerge after the V8 growth stage are a harvesting concern rather than a yield worry. “There is no economic gain for trying to boost yields with weed control after the V8 stage…