Fine-tuning and patience will help avoid damage.

It's crunch time. The weather has been wet and you're behind. Weeds are getting away from you. But the TV weatherman says widespread rain showers are on the way-beginning tonight. And spring rain storms often are heralded by strong winds.

"This is a recipe for desperation, which can lead to mistakes," said Andy Kendig, University of Missouri weed specialist at Portageville. "Especially if a sensitive crop or planting is growing downwind and desperation pushes you to spray in a 20 mph breeze."

Herbicide drift complaints have increased the past few years, Kendig noted. In many cases, the spray drift prompting the complaint could have been prevented.

"Many of these complaints involve glyphosate sprayed on Roundup Ready beans and drifting onto susceptible corn, but there are about as many cases of other products drifting onto other crops," said Kendig. "Remember: soybeans are sensitive to herbicides such as Banvel and Clarity, grain sorghum and non-Roundup Ready corn are susceptible to glyphosate, and cotton is very sensitive to 2,4-D."

The solution to most drift problems is simply to shut down when it's too windy. And-to defy logic-spraying in a dead calm can be equally dangerous, as part of the spray can become trapped in an air inversion and slowly drift over a much larger area.

"Early in 2002, we saw a lot of 2,4-D damage on cotton, although winds were not very strong," said Andy Vangilder, University of Arkansas extension specialist in Clay County. "Rice growers were spraying 2,4-D during an air temperature inversion. A layer of cool, moist air at the surface was trapped under a layer of warmer air above. With these atmospheric conditions, spray particles floated from as far as several miles away in enough concentration to damage cotton.

"We couldn't prove 2,4-D drift is what happened, but there was no other source at the time," Vangilder added. "This happened during one of those freak weather conditions; damage probably would not have been so bad if a little breeze had been blowing."

Drift control technology is fairly straight forward, and it works, observed Kendig. First, make sure the sprayer is in good working order. Calibrate the sprayer before it's pulled into the field, and re-check the calibration each time a different herbicide or tank mix is used. Check spray nozzles and tips to make sure they are putting out an even spray. Beyond tuning up the sprayer itself, Kendig advised paying attention to:

• Sprayer boom height
• Low-drift tips
• Drift-retardant products
• Build buffers

Boom height
"In most cases, the spray boom only needs to be 18 inches above the target (not necessarily above the ground surface)," said Kendig. "Nozzles apply a spray that gives the optimum coverage at that height. But all too often, we see high-clearance rigs that are fogging a herbicide mist 5 feet above the crop or weeds.

"Aerial applicators sometimes get a bad rap, but they cause very few of our drift incidents," Kendig said. "Those guys do nothing but spray, and most of them know their business. When it's windy or a big rainstorm is on the way, they usually don't want to fly."

Low-drift sprayer tips
Air injection (low-drift) nozzles can help reduce drift, too.

"There are several different brands and types to choose from and they work," said Vangilder. "We recommend that sprayers be outfitted with them."

Drift retardants
Drift suppressants are products-usually polymers of some kind-that increase the viscosity of the spray mixture. Most do this by increasing the number of coarse droplets, which are less susceptible to drift.

Cost of drift retardant products varies widely, from about 50 cents per 100 gallons of spray mix to more than $5.

"Most MFA managers use drift retardants at least part of the time. Some managers use retardants in virtually every tank of spray," said Paul Tracy, MFA staff agronomist. "They can help, although these products are no cure-all against drift."

And some compounds and tank-mixes specifically recommend that drift retardants not be used. Others, such as Roundup WeatherMAX specify particular types of drift-control agents.

Build buffers
As Kendig said earlier, much of the drift damage occurs when glyphosate sprayed on Roundup Ready beans drifts to a nearby susceptible crop. It's a big reason some corn/soybean growers are switching to Roundup Ready corn.

But Paul Tracy has reservations about using glyphosate-resistant crops on both sides of a corn/soybean rotation, for a couple of reasons. "For one, following glyphosate-resistant soybeans with Roundup Ready corn gives weeds constant exposure to the herbicide, which could lead to a faster build-up of herbicide resistance in some weeds," he said. "For another, from mid-Missouri north, Roundup Ready volunteer corn in the following soybean crop is a more expensive weed to control. Rather than plant an entire field to glyphosate-resistant corn, I'd consider planting a buffer of 16 to 18 rows of Roundup Ready corn adjacent to where I intended to plant Roundup Ready soybeans, then plant the rest of the field to my normal hybrids."

All of the above drift-management techniques can help prevent herbicide drift damage, said Vangilder. "But they won't give you much protection if you're spraying in a 20 mph wind," he added. "Weather is the main consideration in avoiding spray drift. Sometimes, you just need to shut down and wait it out. Either that, or bite the bullet and use a product with less drift potential-even if it may be more expensive. We have to control weeds, but we also have to be good neighbors."