Reduce your risk of herbicide drift

Reduce your risk of herbicide drift

Drift management is needed this time of year, especially with post spraying of herbicides.

Every spray season, especially at postemerge time, it seems at least half the days or more the wind is blowing, fields are wet and crops are growing closer to the upper height limits of the herbicide label while sprayers sit idle. At the same time, the weeds seem to double daily in size and compete longer with crops for valuable resources like water, nutrients and sunlight.

Related: 'Watch' wind when spraying

BE AWARE: Visit fields to be sprayed ahead of time, and note any sensitive areas nearby. Find out if neighbors are involved in organic production, if honey producers have beehives nearby. Always be aware of nearby plants that are sensitive to the herbicides being applied.

"This season it seems like it has been more challenging than most," says Iowa State University Extension field agronomist Clarke McGrath. "I have to give credit where credit is due; folks are doing a great job so far, phone calls with questions or reports about drift are about where they are on average which is impressive given the small windows for spraying we've had this year."

"Corn postemergence spraying work is moving along and we'll hopefully be moving on to the beans soon if weather cooperates," he notes. "Now is when things can get exciting, as the corn growth takes off there is more urgency to get that done so we may push the envelope. After looking at some of the bean fields, we'll be pushing the limits on some of those post trips as well, so herbicide drift management will become even more critical."

Factors you can control when spraying herbicides
McGrath writes a monthly agronomy column in Wallaces Farmer magazine. He offers the following guidelines and information to help you avoid problems with spray drift.

He says, "Experiencing this phenomenon firsthand in my days of working at a retail farm chemical dealership and custom spraying business, and working with sprayers on farms, has taught me to focus on what you can control when you can get back in the field: application time, boom height, sprayer speed, pressure, nozzle type and wear, carrier rates and additives."

Application timing. Make a good effort to spray when conditions are right. "Having spent nearly a decade trying to keep my fleet of custom rigs ahead of the spray list, I know this is much easier said than done," says McGrath. He says the following considerations might help:

Read the pesticide label.  It will give guidance on wind speeds. A few herbicides also have recommendations for what time of day to spray or not spray. Don't ignore those recommendations. I learned the hard way they mean business.

Pay attention to the wind. You can often prioritize fields by looking at which way and how hard the wind is blowing. A wind meter is invaluable. Use it often during the day and at various places in your field to get a good idea of wind speed and direction. Winds are likely to be calmer in mornings or evenings. During these times you may want to treat fields where crop or weed size is pushing labeled heights, weed pressure is very high (you get better coverage in lighter breezes) or you are near drift-sensitive areas.


The middle of the day is often breezier, so consider spraying in areas where some minimal drift will have less impact. "Almost every application will create some drift, but you should decide if the minimal drift you create will cause problems for a neighboring crop, acreage, gardens, trees or other sensitive areas. And yes, sometimes it is just too windy and the sprayers will have to sit, lessons I had to learn the hard way."

Related: Residual herbicides: key tools in combating hard-to-control weeds

Boom height. Herbicide labels often specify a specific height above the crop that booms must be set. In addition, factor in your nozzle type and the overlap necessary to achieve proper coverage. In general, try to keep the boom on the low side of the recommendations; more of the spray pattern should reach the target that way. Consider using nozzles that have a 110 degree spray angle which allows the boom to be closer to the canopy than nozzles with 80 degree angles.

Sprayer speed. "My advice here is to use good judgment," says McGrath. "I know, you could drive a truck or a sprayer through that guideline. But like any other agronomy answer, "how fast can I spray" depends on the sprayer set-up, field conditions, crop and pest conditions/sizes, weather and the operator. Some machines do a better job than others at higher application speeds. Experience tells me that individual operators are probably in the best position to decide the right speed for their situation. There may be some information in the pesticide product label to offer guidance as well."

Use the right nozzle type and pressure. Nozzle type and pressure go hand-in-hand. The right nozzles usually allow for better coverage and less drift on the lower end of the recommended pressure range. Your local ag chemical dealers are experts in this topic. Most of their sprayers have "dial-a-jets" that give multiple nozzle choices to fit a variety of conditions and products. You may want to talk with them about the benefits of putting "dial-a-jets" on your sprayer.

Herbicide labels also offer good information on nozzle selection and pressure. A trip to a sprayer supply store or browsing one of their catalogs demonstrates that we have better tip choices than ever.

Carrier volume. Higher carrier rates typically allow use of larger nozzles at lower pressures which minimizes drift. Your local ag chemical retailer will be able to guide you through the label information to pick the right volume to match the nozzles, pressure and volume to the products. "I know hauling more water and loading more often is a pain," adds McGrath. "But on calmer days you may be able to drop the volume, the number of gallons per acre, and get more acres per load to help catch up.

Nozzle wear. Worn or partially plugged nozzles not only impact application rates, but may increase drift potential. Each spring many applicators get the sprayer out, fill it with clean water and, during the walk-around inspection, survey the boom as it is spraying to look at spray patterns but they can't always see if a nozzle is worn or slightly plugged.

Many sprayer dealers and farm supply stores carry a tool often called a "tip tester" to check nozzle flow rates. They are easy to use and will tell you if the nozzle has a small restriction and needs cleaning, or is worn (putting out too much flow) and needs replaced.


Using spray additives. Drift reduction and deposition agents don't replace the value of managing the other variables on the list that we can control, says McGrath. But they can offer additional insurance that more of the spray pattern may stay on target. Research and experience show proper use of high-quality drift retardants works well in reducing the formation of the small droplets that contribute greatly to drift. Most ag chemical dealers have worked with enough of these to be able to match the most effective products to your situation.

Related: Don't let weeds steal your yield this year

Recordkeeping. Keep good records of your pesticide applications. It is the law for restricted use products and a good idea for all your spraying work. You never know when you will have to rely on your records.

Follow the rules. If you have questions about rules and legal requirements for spraying farm chemicals in Iowa or requirements for recordkeeping of applications, contact the Iowa Department of Agriculture and Land Stewardship. The Pesticide Bureau at IDALS maintains a couple of online registries that applicators should bookmark: the Apiary List and the Sensitive Crops Directory. Thanks to these registries, pesticide applicators have a resource to help locate these areas before spraying. If your operation falls into the pesticide-sensitive category, you can add it to the registries to help protect your crops or bees from spray drift.

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