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Dryness hampering weed control, farmers say

Jeff Caldwell 05/29/2012 @ 9:57am Multimedia Editor for Agriculture.com and Successful Farming magazine.

The moisture outlook for the parched parts of the Corn Belt and mid-South is improving. While it's likely to slash the heat and drought stress on the region's crops, it may not come in time for farmers to get the most out of the herbicides they've already applied to corn in dry, dusty soils, specialists say.

Temperatures that climbed to near the triple-digit mark over the weekend in parts of the Corn Belt look to be moderating while moisture prospects are improving, says MDA EarthSat Weather senior ag meteorologist Don Keeney.

"Active rains in the northwestern Midwest have increased wetness and slowed corn and soybean planting. Rains are expected to finally return to the southern Midwest and northern Delta over the next few days, though, and more rains are expected in the Midwest next week," Keeney says. "The rains will begin to replenish moisture supplies, and improve conditions for corn and soybean growth."

But, will it come soon enough to get the most action out of postemergence herbicides farmers have already applied to their corn? Though a solid glyphosate application can typically knock down a lot of -- if not all -- weeds in a corn field, if it's put down when it's dry enough to kick up a lot of dust, that efficicacy falls quickly, according to University of Illinois agronomist and weed science specialist Aaron Hager.

"Airborne dust has been shown to reduce the activity of some foliar-applied herbicides, including glyphosate. Greenhouse research conducted at North Dakota State University demonstrated that control of nightshade species with glyphosate was reduced when dust was present on plant leaf surfaces," Hager says. "The reduced phytotoxicity occurred whether dust was present on the leaf surfaces before glyphosate application or deposited there within 15 minutes. If dust was deposited more than 15 minutes after glyphosate application, or if glyphosate was applied 30 minutes before dust was deposited, reduced phytotoxicity was not reported."

And, it's not just research showing these types of results. Farmers around the nation are watching this happen in their fields in areas where rainfall's come up well short of normal so far this spring.

"I had my corn sprayed a week and a half ago with Roundup and Resolve Q. Some of our beans were sprayed last week with Roundup and Classic," says Agriculture.com Crop Talk senior contributor Blacksandfarmer. "I am already seeing a few weeds starting to emerge in the corn. Our last rain was May 6 if you don't count the 5-minute shower yesterday that yielded less than a tenth."

So, what can be done? If you irrigate, you can put on the amount of moisture required for activation. But, keep in mind that water timing can also affect how much of a boost your herbicide gets.

"It would be better if it was 3/4 inch at once (versus 3 1/4-inch rains in successive days)," says Crop Talk senior contributor sw363535. "Some herbicides needs to be pushed to sprouting depth of weeds. Forty points on 3 different days probably would not be as effective."

Beyond controlled watering, there's not a whole lot to do to improve the efficacy of herbicides applied in dusty situations. There are a few spray additives that could help, but the best thing, Hager says, is to keep tabs on any potential weed pressures after you apply herbicide and get a clear picture of how much control you have.

"Spray booms mounted at the front of the sprayer can discharge the spray solution before it encounters dust generated from the tires, but dust deposited on leaf surfaces shortly after application has been shown to reduce herbicide performance. Increasing carrier volume and some spray additives have been shown to reduce, but not eliminate, the deleterious effects of dust," he says. "It is advisable to scout fields that are treated with postemergence herbicides under very dusty conditions to determine the level of weed control."

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