COLUMBUS, Ohio – Growers who got their corn in the ground, but missed their window to apply pre-emergent herbicides to control weeds need not panic.

An early post-emergence application may help prevent yield losses, but be mindful of specific management programs required for no-till and non-GMO corn, says Mark Loux, an Ohio State University Extension weed specialist.

"Because of the recent rains, growers may be facing a wide range of herbicide application challenges," said Loux. "Some probably put herbicides on early with the intention to plant and didn't get the corn in the ground fast enough. That herbicide is now running out and won't give them the four to six weeks of control needed after planting."

That's an easy fix, though, said Loux, who also holds an appointment with the Ohio Agricultural Research and Development Center.

"Throw down some additional residual herbicide at the time of planting," he said.

The bigger issue some growers may be facing is if they didn't apply their pre-emergent herbicides before planting corn. Corn is emerging and so are the weeds.

"That is a more critical situation for the no-till farmers because the decision now becomes what is okay to mix without damaging corn," said Loux. "Eighty percent of corn is Roundup Ready, so a grower can use glyphosate on small corn with 2-4,D and atrazine to control emerged weeds."

With non-Roundup Ready corn, management is a bit tougher.

"You have to come up with the right concoction and not injure corn. What kind of mix will control what's out there that has emerged? What adjuvants can be used? Can the mixture be applied on two-leaf corn? Five-leaf corn? Growers can get guidelines from dealers in those situations," said Loux.

For no-till, non-GMO corn growers, an early post-emergent application may be helpful.

"An early post-emergence application of foliar plus residual herbicides can be just as effective at preventing yield loss due to weed interference, compared with a program consisting of sequential applications of pre-emergence and post-emergence herbicides," said Loux.

However, early post-emergence treatments may not provide adequate "season-long" control of weeds that tend to emerge late, such as grasses, giant ragweed, and waterhemp. They also will not provide adequate control of weeds that are not well controlled by pre-emergence herbicides, such as shattercane, johnsongrass, and burcucumber.

Loux recommends that fields treated early post-emergence should be scouted later in the season to determine if an additional post-emergence herbicide is needed.

He also warns that growers who use pre-emergent herbicides as their early post-emergent application may damage larger corn stalks, so their application choices may be more limited. And if weeds grow larger than corn before an application is made, growers could be looking at yield losses.

"If you've got 2-inch inch corn and 10-inch weeds, you probably already lost some yields," said Loux. "In April when weeds are smaller, it's not much of an issue. But by the time May comes rolling around, it's a jungle in those fields, and you are looking at a big weedy mess."

Additional considerations for an early post-emergence approach are here. For detailed information on weed control, refer to the 2010 Weed Control Guide for Ohio and Indiana.

SOURCE: Ohio State.