August 02, 2005
Lack of rain affected weed control in corn
CARBONDALE, Ill. -- Relying on one application of a pre-emergence weed killer to get rid of weeds in corn will take less time and cost less up front, but it could cut yields at harvest.
“This year a total pre-emergence weed control program was not as successful as normal, primarily because there wasn’t enough rain in the spring,” Southern Illinois University Carbondale weed scientist Bryan G. Young told farmers at the College of Agricultural Sciences annual field day July 13.
Too much rain also cuts the effectiveness of this type of herbicide, but even with just the right amount of moisture, farmers will find it hard to control all weeds with just one pass.
“That does a good job in May but not so well on late-emerging weeds,” Young said.
“We’re lucky if (a pre-emergence herbicide) lasts 45 days, let alone 90 days — and a season-long residual herbicide will not likely ever be registered.”
To get around that problem, some farmers have begun applying a second round of herbicide after the crop pokes through the ground. This costs money and takes more time, but the yield difference makes it worthwhile.
“In late-season control of common cocklebur, for instance, we had 81 percent control with a total pre-emergence program, where with pre and post applications, it went to 98 percent,” Young said.
“That meant an eight-bushel-per-acre advantage, which should pay for the price difference between the two programs.”
In addition, more farmers are turning to corn varieties that can withstand glyphosate, the active ingredient of the popular weed killer Roundup, as a way of trying to get by with one herbicide application.
“There’s more selection now, and there might be more interest because of what they’ve experienced with rescue herbicides (those applied post emergence to get weeds they didn’t kill in the first go-round),” Young said.
Because of grower interest, researchers in 10 Midwestern states have been looking at what it takes to get maximum weed control in Roundup Ready corn. They generally agree that farmers who want to kill weeds all in one go will get the best results if they apply the herbicide when the weeds are at least 6 inches tall, Young said. If applying it twice, farmers will get more consistent control if they hit the weeds before they stand 4 inches tall.
“The single-pass approach is not recommended because the two-pass program will give you the best yield — if you remove them before they get to 4 inches in height,” Young said.
“The problem with this is if you plant your corn in the last week of April, and then when it gets to 4 inches in May, it rains — as it often does — and it’s too muddy to get out in the field, you might sacrifice yield. Timing is critical if you want to protect yields.”
And sometimes that timing needs a little tinkering.
“This year, with the dry spring we had, 4-inch weeds might have been too high,” Young said. “You might have needed to apply your glyphosate at 2 inches.”
Timing becomes a little less critical if growers choose a pre-emergence herbicide and then follow it with an application of glyphosate after the corn comes up. That catches any weeds that escaped the first treatment and the late-emerging weeds as well. In fact, Young said, that might be the best use for Roundup Ready technology.
“We’re using glyphosate to the max right now,” he said. “If we save it to use as a rescue herbicide, we might be able to use it that much longer before weeds develop a resistance to it.”
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