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Balancing Drift Risk and Efficacy

You might as well keep your crop sprayer in the shed if your ultimate goal is zero spray drift.

“That goal ignores how effective you will be at controlling the targeted pest,” says Scott Bretthauer, University of Illinois Extension application technology specialist. 

Rather, he says, your goal is to strike a balance between the risk of off-target drift and good efficacy that accomplishes the mission of crop protection from weeds and other pests. Here are seven points to remember as you strike that balance.

1. All spray nozzles produce a range of droplet sizes.
The volume median diameter (VMD) tells you a nozzle’s median droplet size, with half of the spray volume contained in droplets bigger and half smaller. If the VMD is 100 microns (about the diameter of a human hair), that means half of the spray volume came out in droplets less than 100 microns, and those are most prone to drift. You may also hear about a nozzle’s droplet size spectrum (the range of sizes it produces) and its percentage of small droplets (usually those are between 100 and 200 microns).

2. Droplet size influences coverage and spray drift.
Small droplets under 200 microns provide better spray coverage at a given volume of spray material because there are more of them. Because they are smaller and lighter, they can more easily blow off target. A study from Ohio makes a good illustration, says Bretthauer. It shows that droplets of 200 to 300 microns have little or no drift in winds up to 10 mph. A droplet of 150 microns will move about 5 feet off target in that speed of wind and at a boom height of 18 inches. A 50-micron droplet may move 50 feet under those conditions.

“Wind definitely influences drift, but it is very dependent on droplet size,” says Bretthauer.

3. Stable air is the scariest.

This is the issue of air inversions, a phenomenon not well understood by many sprayer operators. It happens when very stable air is trapped near the ground (fog is the best example). There is little wind and little vertical air lift.

Inversions happen in the late afternoon, evening, and early morning, when there is no sunlight to heat the air and to create lifting and mixing. The problem inversions present with crop spraying is that if spray material in small droplets is caught in one, it hangs there and can move off target as the air mass drifts over the countryside. Some of the most profound and disastrous examples of off-target movement to crops have happened not in the presence of high winds, but rather in an air inversion.

One solution is to apply what Bretthauer calls the 3°F. rule. “Most air inversions happen between evening and early morning,” he says. “If you’re going to spray in the morning, note the air temperature at morning low and wait for the sun to warm it by 3°F. That should break the stability of the air and reduce the potential for spray drift.”

4. Whenever possible, lower the boom.

In the same Ohio work mentioned earlier, researchers measured spray drift at various boom heights. With 300-micron droplets, there was not much drift as the boom varied from 6 inches up to 3 feet above the target crop. However, at 100-micron droplets and smaller, drift was as far as 50 feet off target with the boom 3 feet high.

“The effect of lowering the boom is dramatic,” says Bretthauer. “Anything you can do to lower it reduces the potential for drift.”

5. Smaller droplets mean more of them.
As droplet size is reduced, it takes many more to deliver the same liquid volume of spray material. Bretthauer says if you go from 500 microns down to 250 microns, it takes eight times as many droplets to achieve the same volume of spray. At 125 microns, it takes 64 times the number of droplets.

“Sometimes, the chemistry determines the importance of this,” says Bretthauer. With glyphosate, for example, big droplets or small droplets do not make much difference in terms of efficacy. With an herbicide like paraquat, the more droplets, the more absorption. “As a general rule,” says Bretthauer, “you want to use the biggest droplet size you can get away with and maintain efficacy.”

6. There’s new nozzle technology.
Most of it is aimed at reducing the number of fine particles, the ones that drift in the wind or hang in an inversion. The newer air-induction nozzles (those with larger exit orifices and those with turbulence chambers) are designed to reduce the pressure as spray material leaves the nozzle. In turn, this produces fewer small and fine droplets.

“Your goal is to find a nozzle that gives you the best of everything: the fewest fine droplets and the best efficacy,” says Bretthauer.

In general, he says, nozzles with smaller orifices and wider fan angles produce finer droplet sizes.

7. Solutions can change flow rate and droplet size.
As you add products to spray mixes, be aware that it can greatly change the way the material moves through nozzles and can change droplet size, says Bretthauer.

“Calibrate your nozzles so you know exactly what volume of material you are applying,” he says.

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