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Corn+Soybean Digest

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, says Bill Johnson, a Purdue University weed scientist. In addition to their unsightliness, the weeds could take valuable nitrogen (N) 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 says.

"In a system where we don't use soil-applied herbicides we're going to have weeds emerging with the corn," Johnson says. "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.”

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

"We find that early in the year grass weeds take up nitrogen at approximately the same rate as corn," Johnson says. "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 N to cut into corn yields, Johnson says.

"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 says. "What we have seen in our research across a number of different weed species is by the time the weeds have between 10 lbs. and 20 lbs. 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.”

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

"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 says. "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 says.

To learn more about early season weed control in Roundup Ready corn and weed uptake of N, 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, located at

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 at

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