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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."

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