Postemergence weed control programs

It's about time for many of the Delta's cotton growers to begin making plans to apply an over-the-top postemergence weed control treatment to their crop. And, for those growers who planted Roundup Ready cotton, proper application timing is crucial to the success of their herbicide treatment.

“Remember, when temperatures are favorable, cotton can really put on some leaves in a hurry, so be certain to make those over the-top-applications in a timely manner. It's also a good idea to prepare for those applications before they need to be made,” says Steve Kelly, weed specialist at the Louisiana State University AgCenter in Winnsboro, La.

The LSU AgCenter recommends that Louisiana growers make two consecutive over-the-top applications of glyphosate before the fifth cotton leaf is the size of a quarter. The first application should be made when the cotton plant is between the cotyledon and one-leaf stages. The second treatment should be made about 10 days, or two nodes of growth, later.

In Mississippi, state recommendations call for cotton growers to limit their postemergence glyphosate applications to the period before the cotton plant reaches the four-leaf stage.

Kelly says he tells growers to make the second glyphosate application to Roundup Ready cotton, whether or not they think they need it. “I tell most people not to even look to see if they need the second application. It's much better to pull the trigger and make the application, regardless of whether you think you need it or not. I believe that prior research and on-farm experiences will bear this out, especially if morningglories and hemp sesbania are present.”

“Most growers run into trouble when they try to cut corners and delay or do without one of these treatments. If these two applications are made automatically, we can keep from getting into a situation where a later-than-desirable application has to be made in order to salvage a crop,” he says.

While glyphosate provides excellent control of many annual broadleaf and grass weeds, including johnsongrass, control of morningglory and hemp sesbania is probably its weakest link, according to Mississippi Extension cotton specialist Charles Ed Snipes. “If these are problem weeds in your Roundup Ready cotton field, multiple applications of glyphosate or glyphosate with Staple may be necessary to provide acceptable control of the two weeds.”

Kelly advises cotton growers to scout their fields before making any early-season weed-control treatments to determine what weeds are present and whether a tank-mix treatment is needed. “A little Staple in the mix can help out on morningglories and sesbania. If grasses are your problem, glyphosate alone should do a sufficient job,” he says.

He adds, “There are a lot of generic glyphosates on the market this year. Not all of them are labeled on Roundup Ready cotton, so be sure you choose one that is labeled for this use. With all the guarantees and marketing schemes aside, most of these do an equal job of controlling weeds.”

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