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10 tips for better crop spraying

01/06/2012 @ 2:44pm

Spraying pesticides on your fields is one of those jobs where the margin for error is pretty narrow, both in terms of accomplishing the mission and protecting neighboring fields. Mark Hanna, Iowa State University Extension agricultural engineer offers these 10 tips for more effective and safe spraying.

1. Small drops take time to hit target.

A tiny droplet 100 microns in diameter (about the diameter of a human hair) takes 11 seconds to fall 10 feet. At 50 microns, it takes 40 seconds to fall that far, because of the drag that air friction puts on them. That's a long time for a wind current to move that droplet to an unintended target.

"Those tiny droplets really slow down when they leave the nozzle," says Hanna. "The other important part of that is that you may think you can crank up the pressure and drive those small droplets down into the crop canopy. It doesn't work because they slow down so fast."

2. Droplet size depends on pressure.

As sprayer pressure increases, droplets get smaller. Decrease pressure and they get larger. Change the pressure as you go through a field, and droplet size changes, too. That changes how the spray material moves in air, and may increase the likelihood of spray drift.

This explains why it's good practice to slow down when you spray near neighbors' gardens or crops. As you slow down and the controller reduces nozzle pressure, droplets grow larger and are less likely to drift.

3. Small droplets dry quickly.

That’s particularly true during warmer temperatures that commonly occur during post-emergence applications. The water in a droplet under 150 microns in size can evaporate in a few seconds in the right conditions. Wind can then easily move the chemical residue, creating a drift issue.

In contrast, it takes two to three minutes for a bigger droplet to evaporate, meaning it reaches the target before significant water loss occurs.

4. Nozzles produce variety of nozzle sizes.

Your sprayer nozzles may be identical, but droplet sizes are not. Each nozzle produces a range of droplet sizes. If you spray a medium droplet of 225 to 325 microns, 5%-10% may be fine droplets of 150 microns or less, and 5%-10% may be large droplets of 450 microns or more. Those small ones are the most likely to drift off target.

"You may think you're spraying at a droplet size that won't drift, but be aware of this normal variation," he says. And, don't be reluctant to change your nozzles as they age and lose performance characteristics.

"A new nozzle might cost $5, and a 90-foot boom with nozzles on 20-inch centers will have 54 nozzles," Hanna says. "That's $250 to change them all, but think about it in terms of the hundreds of acres you're going to put them over. On a cost per acre basis, that's not expensive."

5. Weather impacts spray drift.

Drift is greater at warmer temperatures, and is greatly impacted by boom height. "At a warm temperature like mid-80 degrees, a 10 miles per hour (mph) wind can create spray drift distance similar to a 15 mph wind at 50 degrees," says Hanna. And, a boom height of one to two feet above the crop canopy will usually have little drift. But at three feet, drift distance goes up significantly.

6. Remember coverage and efficacy.

You can increase droplet size to reduce spray drift, but remember to consider spray product coverage and efficacy. You want to get the spray on the target area, and larger droplets can reduce your ability to do that. Here's why: If the droplet size is 400 microns and you are spraying 15 gallons of product per acre, it's putting out about 270 drops per square inch. This is often adequate for systemic pesticides (the kind that travel or translocate within the plant). Reducing droplet size to 300 microns (at 15 gallons per acre) increases coverage to 640 drops per square inch. The extra drops per square inch can improve performance of a contact pesticide (impact only what they land on).

Hanna points out a research study in cotton where they tested coarse, medium, and fine spray droplets. "Droplet size didn't make much difference in coverage at mid-plant," he says. "But at lower levels of the plant, the fine droplets did not penetrate and cover the target as well. That proves again, you can't drive those very small droplets down into the canopy."

7. Read the label.

As for fungicides sprayed on soybeans, it's easier to get coverage on the mid-plant than the plant bottoms. But in this case, droplet size doesn't seem to impact it; fine, medium, and coarse droplets achieve about the same coverage. "Medium size might win in that case," says Hanna. "A good starting point is probably 250 micron droplets for all classes of sprays." In fact, he continues, some newer pesticides put that right on the product label - 250 to 350 microns, along with a desired application rate such as 15 gallons per acre.

"Read those labels, they give you tips for good spraying techniques," Hanna says. "It may even help later if you have an efficacy issue. You can say you followed their label."

8. Venturi nozzles balance efficacy and drift.

The newer Venturi-style nozzles offer an advantage to older styles: You can get larger droplet size at a given pressure. There's a small hole on the Venturi nozzles that draws air into the liquid flow. This can produce air bubbles in the spray stream, giving larger droplets and less potential for drift in some circumstances. Newer styles of Venturi nozzles operate at lower pressures, and although droplet size is slightly smaller than older Venturi styles, they may help maintain a balance between efficacy and drift, depending on application needs.

9. Manage rate controllers.

This will help you maintain consistent coverage. In some cases, you may double spraying speed from 6 mph to 12 mph. To apply the same spray volume, you have to double the flow rate because you're covering twice as much ground. You have to increase pressure by a factor of four to produce that volume.

"If you were at 20 psi (pounds per square inch) at the slow speed, now you have to go to 80 psi at the higher speed. Some equipment and nozzles aren't designed for that," says Hanna.

This pressure increase can change the droplet size spectrum. You could go from a coarse spray to a medium, or even fine.

"Controllers are great, you just need to understand how they work and manage them for the desired outcome," Hanna says.

10.You’re in charge.

"It's not just the nozzles, booms, controllers, or other pieces of hardware," says Hanna. "It's you and your knowledge of your equipment and sprayer characteristics. We have some excellent new equipment out there, but they are all just tools and can be misused. Understand them, know what you are trying to do, and you can minimize the potential for spray drift."