Last year, several southeastern states saw incidents of off-target damage to crops from dicamba. As of October, Missouri had 124 off-target complaints; Tennessee, 47; Arkansas, 28; and Mississippi, 12. "Even in our 12, that was a fourfold increase in the number we had in previous years relative to dicamba," says Dan Reynolds, professor of agronomy at Mississippi State University.

However, as herbicide resistance becomes more widespread, auxin herbicides like dicamba and 2,4-D and their respective resistant corn and soybeans will be key tools in growers' weed control programs. "So, mechanisms of mitigation are going to be very important," Reynolds adds.

Recently, Reynolds and Greg Kruger, associate professor of agronomy at the University of Nebraska-Lincoln discussed a study conducted in Nebraska and Mississippi over the last several years involving the use of hooded spray booms to mitigate the amount of off-target movement.

This study compared movement from fine, medium, very coarse and ultra-coarse droplets with an open boom versus a Redball hooded boom to minimize drift. Using these droplet distributions, the study used a 642E Redball hooded three-point sprayer in a 30-foot width, specially designed for research purposes by Willmar Fabrication. This was compared to Willmar's 642 open-boom sprayer with no hoods.

"When we talk about particle size, there's always a trade-off between off-target movement and coverage, and certainly that relationship's going to be different for different products," says Kruger. "What we're seeing is the industry is really moving toward using coarser and coarser spray droplets."

In the study, researchers made a 600-foot spray pass, perpendicular to wind direction, which was blowing from 7 to 9 miles per hour. Spray material was marked with fluorescent dye, and Mylar cards were placed downwind to intercept the spray deposition and determine relative drift and dilution and different distances.

Reduced drift potential
Here's a look at some key findings:
• With fine droplets, the hoods reduced the amount of relative drift at seven-foot distance down to about 20%.
• With a medium droplet size, relative drift was reduced to just over 60%. With the addition of the hooded boom, relative drift was further reduced to about 20% at a 7-foot distance.
• With very coarse spray droplets, relative drift was reduced to less than 20% at 7 feet. With the addition of a hood, relative drift was reduced even further, to less than 10%.
• By reducing the droplet size to ultra-coarse, there was a nearly 85% reduction in drift — about 15% relative drift at 7 feet. With a hood over the top, drift was reduced to less than 5%.

"On all these droplet sizes, the Redball hooded boom sprayer reduced the amount of particle drift we found out there based on the dye we had on those cards. As the droplet size increases, that magnitude of difference between the Redball hooded spray boom system and the open-boom system was not as great," Reynolds says. "We did see a decrease, particularly with the fine and medium droplets on the sprayer. This gave us a lot of opportunities to achieve maximum coverage, while minimizing the particle drift that's there."

"Even when we coupled the Redball sprayer hoods with the ultra-coarse droplets, we were able to minimize or reduce the drift from the nozzle," adds Kruger. "It's a part of a system and a very valuable tool, especially when we talk about spraying very highly active compounds around sensitive areas. I think the key is to remember the application systems are complex. There's not going to be a silver bullet for any technology."

Worth the added cost
Steve Claussen, president and CEO of Willmar Fabrication, notes hooded spray booms also saw more consistent coverage, especially when dealing with taller weeds. "With hoods, an added benefit is you can run at the proper height, and if you have taller weeds, it lays them over," Claussen says. "As the weeds go underneath, the whole plant gets covered before it comes back up."

The cost to fit a generic spray boom with Redball hoods is about $200 per foot. A 40-foot 642E sprayer, including tank and pump, will run about $25,000. Redball also offers the SPK645 — a kit to retrofit self-propelled sprayers. On a 90-foot sprayer, the SPK645 kit would add 60 feet of hoods at an average cost of about $300 per foot, for a total cost of $18,000.

When it comes to minimizing drift, Kruger and Reynolds say the benefit is worth the added cost. "When we have drift, that's lost product on the field. There's a cost to losing that product. If there's incidence of drift, the cost of either settlement or legal ramifications would be so high it wouldn't even be comparable," Kruger says. "Anytime we have a tool to help us reduce drift, it's valuable."

Learn more about the study results by contacting Kruger at [email protected] and Reynolds at [email protected].