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Early-season weeds costly in corn

When it comes to controlling early season weeds in their corn crop, growers have two choices: pay now, or pay later — but it will cost you more later.

According to Extension corn specialist Erick Larson at Mississippi State University in Starkville, Miss., a timely burndown herbicide application followed by a pre-emergence tank-mix of atrazine and a grass herbicide can save growers both time and money.

“Weed competition early in the season will reduce corn yield potential considerably. Also, those producers who do not adequately control early season weeds with a pre-emergence herbicide program, will likely spend much more money trying to control the same weeds with a postemergence herbicide,” he says.

Larson stresses the need to start the season with an early burndown treatment, which he says promotes warmer, drier seedbeds during the spring and encourages earlier planting. “Early planting helps corn avoid stress associated with mid-summer drought, and while irrigation can help alleviate water stress, it does not override the importance of early planting.”

Timely burndown herbicide treatments made four to six weeks before your expected planting date is important for several reasons. “Glyphosate-based herbicides are fairly slow to kill existing weed vegetation, and an early burndown allows the soil to dry out, and warm-up more quickly,” he says. “If there is a lot of green vegetation on the soil surface it inhibits the solar radiation from reaching the soil surface. Enabling the soil to absorb more solar radiation from the sun prior to planting allows the soil to reach a more moderate moisture level, widening your planting window and promoting corn seedling vigor.”

Mississippi State University research has shown that early season burndown treatments also produce higher crop yields than burndown treatments applied closer to planting time, Larson says.

In addition, the Mississippi Department of Agriculture and Commerce is again this year restricting burndown herbicide applications by aircraft as a pre-emptive strike against off-target drift. This year's restrictions will be similar to last year, with Delta counties divided into two zones north and south of Mississippi Highway 8. Aerial applications will be prohibited in the southern zone from March 15 through April 30, and in the northern zone from March 25 through April 30, except by permit from an authorized Bureau of Plant Industry employee.

“If at all possible, I encourage producers to put out any burndown materials before that restriction takes effect,” Larson says.

At planting, or shortly after crop emergence, Larson recommends growers follow the standard pre-emergence weed control program of atrazine tank mixed with a pre-emergence grass material.

“By the time you begin planting your corn crop, you want to have all winter weed vegetation, including things like rye grass and dock, controlled,” he says. “Targeting early-season weeds at planting with atrazine will control sicklepod, morningglories, and other broadleaf weeds. Adding a grass material to your pre-emergence weed control program will target signalgrass, crabgrass, barnyardgrass and other problem grasses.”

While there are several different grass materials available to corn growers, Larson suggests growers look for those herbicides containing the active ingredients acetochlor (Harness Xtra, Surpass and others), metolachlor (Dual II Magnum), pendimethalin (Prowl), or dimethenamid (Outlook or Guardsman Max).

An effective early season weed control program is especially important, he says, because corn is a grass crop. “There are not a whole lot of herbicides available to control grass weeds after the corn plants reaches 2 or more inches in height. The postemergence herbicide products are also a lot of more expensive, at least as compared to atrazine,” Larson says.

Transgenic corn hybrids are increasingly becoming more prevalent in the Delta, with both Roundup Ready corn and Clearfield, or Imi-tolerant, corn planted on a large acreage across the region.

“The two herbicide chemistries in these transgenic systems do offer some postemergence weed control advantages over many of the conventional herbicides,” Larson says. “Glyphosate (Touchdown, others) is extremely effective against most grass species, although one of its weaknesses is morningglory control. Atrazine complements glyphosate by providing some residual activity, particularly on morningglories.”

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