Watch these weeds

Wintry months of historically high and frequent rain, sleet and snow have provided some of the best prospects in years for crops in Texas and the Southwest this spring. But it also gave some mighty good prospects for weeds to run rampant.

Todd Baughman, Texas AgriLife Extension Service agronomist, Vernon-Chillicothe, says wheat growers don’t want to let weeds rob them of what could be one of the best grain harvests in years. He says growers can expect at least a half-a-dozen weeds to be especially challenging in wheat fields down the stretch toward harvest, which is usually late May or early June in north Texas and southern Oklahoma. These include mustard weeds, kochia, tumbleweeds (aka, Russian thistle), henbit, sunflowers and wild oats.

Key Points

• Months of unusually frequent rain and snow is bringing on the weeds.

• Growers need to watch for weed flush as wheat nears harvest.

• Cotton could face greater-than-usual weed challenges this season.


The good news is growers have many tools to control the weeds. And most herbicide options can be used up to the boot stage of wheat growth.

Cotton weeds

Many producers in Texas grow wheat and cotton. So while they are watching weeds just before wheat harvest, they also should be alert for weeds being worse than usual in cotton this spring and summer growing season.

Baughman says he expects marestail (aka, horseweed), kochia and tumbleweeds to be among some of the early weeds. Later during the growing season, Palmer amaranth (aka, pigweed or careless weed) and morningglory can really damage a cotton field.

Baughman says, fortunately, some farmers still use a preplant incorporated, or so-called “yellow herbicide,” in addition to postemergent herbicides, which may be especially important with the subsoil moisture available this year. “And we expect lots of cotton in 2010,” he notes. “Some grain will go back to cotton. If guessing, I’d say we’ll have a 10% increase in cotton acres this year.”

Joe Armstrong, Oklahoma State University weed control specialist, Stillwater, says he hears some concern over herbicide-resistant weeds.

The OSU specialist says in addition to watching for herbicide resistance by pigweeds in cotton, he also sees some resistance by ryegrass in wheat, and that resistance appears to be increasing.

Tyron Spearman, executive secretary of the National Peanut Buying Points Association, Tifton, Ga., also reports pigweeds have become so resistant to herbicide in Georgia that many producers have reverted to a two-step process of having crews remove weeds by hand and haul them out of the field to burn. One pigweed contains 500,000 seeds, and if left in the field, just a few can produce incalculable offspring.

After so much rainfall, another weed worry is volunteer cotton emergence, which is a perfect host for boll weevils.

Gaylon Morgan, Texas AgriLife Ex-tension state cotton specialist, College Station, says 2,4-D herbicide is still used to zap volunteer cotton plants. David Drake, Texas AgriLife Extension agronomist, San Angelo, agrees 2,4-D is a good product but emphasizes control is easier in the five- to eight-leaf growth stage, than the 10- to 12-leaf stage.

New technologies help

Low-drift nozzles can reduce the proportion of very fine spray droplets that are susceptible to drift. “These low-drift nozzles are really having a revolutionary impact,” says Tom Wolf, a research scientist with Agriculture and Agri-Food Canada, and a member of the Weed Science Society of America. “Research shows we can reduce the spray that drifts away from its target to less than 0.5% of the applied amount. That’s a decrease of more than 80% compared to conventional sprays.”

But it’s not perfect. Not all weeds and all herbicides respond the same way when changes are made to reduce the number of very small spray droplets.

“It can be tough to provide practical recommendations to applicators on diversified farms,” Wolf allows. “But scientists are meeting that challenge by communicating their research findings to herbicide manufacturers and applicators alike. We are finding low-drift sprays can be used successfully in the majority of situations.”

Multiple strategies for controlling drift

Robert Klein, a Weed Science Society of America member and crop specialist at Nebraska’s West Central Research and Extension Center, suggests multiple drift control strategies to achieve best results. Here are eight techniques he recommends:

• Increase spray coarseness. Switching to low-drift nozzles reduces drift. For traditional nozzles, lowering pressure increases droplet size and reduces drift. There is a fine line, however, between droplets that are too small (causing excessive drift) and too large (providing insufficient coverage). Look to your nozzle manufacturer for guidance. Low-drift nozzles typically require a higher pressure to operate.

• Gauge the weather. Environ-mental factors are critical to controlling drift. Klein cites wind speed. When wind speed increases, so does the potential for drift downwind of the sprayer. Today’s ultrasonic weather sensors can help, which measure air temperature, humidity, dew point, barometric pressure, wind chill, wind direction and wind speed. The compact units can mount inside a sprayer cab, offering the applicator continuous weather data.

Control the flow. Modulated flow controls reduce drift by rapidly pulsing each spray nozzle on and off. This allows pressure and droplet size to remain constant as the sprayer moves across the field at various speeds.

Rev up the air speed. Air-assisted sprayers emit a high-velocity, downward air stream that pushes the spray droplets directly onto the targeted weeds or treatment area, reducing drift.

• Create a buffer. Untreated zones around fields can serve as buffers that keep drift away from off-target sites.

Add drift retardants. If using a traditional sprayer, special additives reduce the number of fine droplets. Make certain to select an additive designed for the type of nozzle and herbicide used.

• Hood your sprayer. By mounting hoods around the nozzles on a sprayer boom, you can create a physical barrier that reduces spray drift.

Monitor the boom height. Extension specialists recommend keeping booms close to your weed target to decrease downwind drift. Follow the manufacturer’s instructions to maintain the uniform spray pattern and coverage needed to control weeds.

Jan Hatler, Texas Department of Agriculture inspector, cautions growers that no matter how they apply a regulated pesticide, it must be consistent with the product’s label.

You must keep records of your application for two years, should you get a visit from an inspector. Those records must include what was applied (brand name), in what amount, when, for what, the specific field location and the weather conditions, including the local wind speed at the time of application.

The local wind speed means at that field — not what some weather station 80 or 100 miles away says the wind speed to be.


05103847A.tif

NO-GOOD MUSTARD WEED: Hare’s mustard can be an especially pesky weed in a wheat field.

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HOLD THE MUSTARD: Summer mustard weed grows rapidly with some warm days and can spread quickly.

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WILD OATS TOUGH TO TAME: Wild oats can be one of the most prolific weeds in a wheat field.

This article published in the May, 2010 edition of THE FARMER-STOCKMAN.

All rights reserved. Copyright Farm Progress Cos. 2010.