Specialist: Get ready for weeds in early Roundup Ready corn

As more producers choose Roundup Ready corn, they might have to become accustomed to the unfamiliar sight of weeds at crop emergence, said Bill Johnson, a Purdue University weed scientist. In addition to their unsightliness, the weeds could take valuable nitrogen away from the corn, resulting in yield losses.

In many conventional corn systems, producers apply atrazine-based herbicides before the crop is planted, thereby keeping weeds at bay until the corn is several inches high. With Roundup Ready corn, some farmers are likely to abandon these preplant, soil-applied herbicides and spray Roundup later in the season, Johnson said.

"In a system where we don't use soil-applied herbicides, we're going to have weeds emerging with the corn," Johnson said. "If there is nitrogen in that field, weeds will utilize it and enhance their growth rates early in the growing season. The weeds could become very competitive with the corn.

"Essentially, we could be fertilizing the enemy, giving it a competitive advantage and allowing it to grow more quickly, which may cause it to outgrow the Roundup technology that we're using to control it if we have a prolonged period of wet weather after planting that keeps the sprayers out of the field."

Research conducted by Johnson indicates that annual grass weeds such as foxtail, crabgrass and fall panicum are adept at absorbing nitrogen.

"We find that early in the year, grass weeds take up nitrogen at approximately the same rate as corn," Johnson said. "That continues until the grass weeds are about 4 inches tall, and then the weeds go into a period where they accumulate nitrogen very rapidly — more rapidly than corn at that point. By the time the weeds are a foot tall, if you look at the amount of nitrogen in grass on a per-area basis, they can contain three times as much nitrogen as the corn can at that time."

Weeds don't have to take up much nitrogen to cut into corn yields, Johnson said.

"It does appear that there is a relatively low threshold level of nitrogen that can be present in grass weeds to cause yield losses," he said. "What we have seen in our research across a number of different weed species is by the time the weeds have between 10 pounds and 20 pounds of nitrogen per acre in their above-ground biomass, that appears to be a threshold level at which we start seeing yield reduction in corn. We've observed that across foxtail, shattercane, waterhemp and giant ragweed infestations.

"If you have that much nitrogen in the above-ground biomass, you probably have about that much in the below-ground biomass as well. So we may be representing only about half the amount of nitrogen that's in the weeds."

Farmers who plant Roundup Ready corn need to make timely applications of the herbicide in order to prevent possible yield losses, Johnson said.

"A Roundup Ready system can be very effective, but the importance of the proper application timing of glyphosate — or Roundup — is going to be much more important in corn than it is in soybeans," he said. "In corn, if we miss that optimal application window by just a couple of days, we're looking at a 1-2 bushel yield loss per day."

Annual grass weeds should be controlled before they reach 4 inches in height, or about 23 days after planting when corn is at the V2 to V3 leaf stages, Johnson said.

Other strategies for controlling early season grass weeds in Roundup Ready corn include:

• Continuing to use soil-applied residual herbicides. "For atrazine premixes, utilizing anywhere from about a one-half to full labeled rate at or slightly before planting and then following up with glyphosate post-emergence, is very effective," Johnson said.

• Performing two post-emergence applications of Roundup. "Target the first application when grass weeds are 3-4 inches tall and make a second application before corn has eight collars or is 30 inches tall," Johnson said. "This strategy is less desirable than using a soil-applied herbicide followed by glyphosate because we're putting a lot of selection pressure out there for glyphosate-resistant weeds. This strategy also depends on your ability to get over your corn twice with a sprayer."

Farmers should scout their fields 2-3 weeks after an initial post-emergence herbicide treatment, Johnson said. "Get into the field and pull back some of the residue and see if weeds are germinating," he said. "Remember that the first application of glyphosate will not provide any residual activity on weeds unless it is tank mixed with another herbicide that has residual activity."

To learn more about early season weed control in Roundup Ready corn and weed uptake of nitrogen, read Johnson's Extension publication, "Nitrogen Accumulation by Annual Grass Weeds in Roundup Ready Corn Production." The publication can be downloaded from the Purdue Weed Science Web site.

For more information on post-emergence weed management in corn, refer to pages 37 and 38 of the 2006 Weed Control Guide for Ohio and Indiana. The guide can be downloaded.

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu

Source: Bill Johnson, (765) 494-4656, wgj@purdue.edu

Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
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