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Rain delays & weed control challenges

Jeff Caldwell 05/15/2013 @ 7:42am Multimedia Editor for Agriculture.com and Successful Farming magazine.

Much of the Corn Belt has seen a return to better corn-planting weather over the last few days, with temperatures not only reaching seasonal levels, but surpassing them by double-digits on Tuesday. And, while the planting window looks to remain open for the next couple of days, the delays are far from over.

"The drier pattern across the Midwest and Delta so far this week has allowed corn and soybean planting to progress very well, while spring wheat planting has improved in the northern Plains. However, rains should return to central Midwest over the next few days, which will slow planting there a bit," says Don Keeney, senior ag meteorologist with MDA Weather Services. "Rains should also pick up in the northwestern Midwest and northern Plains late week and over the weekend, which will stall planting there."

It's a maddening addition to a planting season already chock-full of weather delays. And, it's causing a backlog of fieldwork that, at this point, experts say must take a backseat to planting. Applying herbicide is one of those jobs.

But, you're not automatically setting yourself up for a season of fighting untreated weeds if you can't get your preemergence herbicides on before you plant corn. Many such products can still be put down after the crop's in the ground, says Ohio State University Extension weed scientist Mark Loux.

"One of the problems with a wet start to the season, when planting gets compressed into short periods, is the lack of time to apply preemergence herbicides and 28% before corn emerges. Most preemergence corn herbicides can be applied to emerged corn, and some of them have enough foliar activity to control small, emerged weeds without the need to add postemergence herbicides," Loux says in a university report. "It’s also possible to mix in some other POST herbicides such as Impact, dicamba, 2,4-D, Capreno, etc to control emerged weeds, instead of glyphosate or Liberty."

If you do plan on applying a preemergence herbicide after the crop's emerged, though, don't waste too much time before you get started. A lot of products have specific limitations on when they can be applied to growing plants.

"Only a few herbicides can be applied using 28% as the spray carrier. These are Degree, Degree Xtra, and Bullet. Degree Xtra and Degree can be applied in 28% on corn up to 6 inches tall, when air temperatures are less than 85 degrees F. Bullet can be applied up to 5-inch corn with the same temperature restriction. Expect some leaf burn from these mixtures. All other herbicides must be applied using water as the spray carrier," Roux says. "There is usually a maximum corn size specified, which can be based on growth stage or corn height. This can be as small as the V2 stage for some herbicides, such as Corvus and Balance Flex."

And, don't just pay attention to the size of your corn plants; keep an eye on any weeds starting to poke through, too. They need to be knocked down fairly quickly.

"Based on our research with this type of approach to herbicide management, herbicides should be applied when weeds are less than about 2 inches tall to ensure that they have been prevented from causing yield loss," Roux says.

But, in general, pay close attention to the label of the product you're applying; some preemergence and soil-residual herbicides either can't knock down emerged weeds or can damage young corn plants, says University of Illinois weed scientist Aaron Hager.

"Most, but not all, soil residual herbicides can be applied after corn has emerged. Products such as Balance Pro, Fierce, Prequel, Sharpen and Verdict must be applied before corn begins to emerge; applications of these products to emerged corn can result in significant corn injury. Be cautious about applying a soil-residual herbicide in UAN carrier if corn has emerged as this can increase the potential for corn injury," he says. "Several soil-residual herbicides can control small, emerged summer annual weeds. Additional management procedures (such as the addition of a herbicide that has postemergence activity) will be needed for those products that lack the ability to control emerged weeds. Be sure to consult the respective product label for tankmix and additive recommendations.

"Labels usually indicate a maximum corn growth stage beyond which applications should not occur. These growth stages can range from as early as two leaf collars to as late as 40-inch tall corn, so be sure to consult the respective product labels," Hager adds. "Products containing atrazine must be applied before corn exceeds 12-inches tall, although the labels of some atrazine-containing products specify a smaller corn height."

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