Uniformity, environment and water all factor into efficiency
There is no perfect day to spray, plain and simple. Farmers are often backed in a corner, avoiding wind one day only to face rain the next. Spraying delays mean weeds grow larger and become more difficult to control with herbicides.
Before heading out to spray, make sure all tips and screens are appropriate for the job across the width of the boom, particularly on equipment that’s new to your operation, says John Byrd, weed scientist, Mississippi State University. Regardless of sprayer brand, uniform tip sizes and screens are necessary to properly filter particles. Variations in pressure can directly affect flow rate through a tip.
Strike a balance between application volume and coverage.
“You want to produce a relatively small droplet, but not so small it increases the potential for off-target movement,” Byrd says. “I know a lot of people want to drop below 10 gal. per acre to maximize ground covered. I’m a bigger fan of a minimum 12 gal.to 15 gal. per acre. Test the sprayer and make a pass with water to be sure you’re getting a good wetting pattern.”
Spraying is always accompanied by drift concerns, so it’s imperative to monitor environmental conditions such as wind speed and direction. A medium droplet size allows for good coverage but reduces drift risk, Byrd says. Try 01, 02 and 03 size nozzles.
Issues arise from multiple sources on a sprayer, but what comes out of the nozzle matters most.
Most chemical labels recommend a spraying window of 2 mph to 10 mph—a relatively narrow range.
“Since wind is a big issue, it’s important to keep the boom as close to the target as practical. Even without wind—less than 2 mph—it may be a case of temperature inversion as air away from the surface gets warmer,” Byrd explains. “When inversion occurs, there’s a greater potential for droplets to move horizontally instead of falling with the pull of gravity, creating a greater possibility for off-target movement.”
Water constitutes most of what is applied when spraying—it’s the foundation of a successful spray pass. “Water is not on the sideline watching the game, it’s one of the players,” says Fred Whitford, director of the Purdue Pesticide Program.
Sometimes pesticides don’t control weeds like they should—and one of the reasons is tied to water quality. Water might have a high mineral content, such as calcium and magnesium, which binds with a herbicide product and actually dilutes the application rate. If acid or alkaline (pH) is out of balance, it can lead to chemical breakdown and less active solution. In other cases, farmers might fail to follow the label and use a surfactant to spread the pesticide over a leaf or an oil to bolster penetration through the waxy leaf cuticle.
“Lots of times, it’s not that a chemical isn’t working properly, it’s just the water isn’t ready to do its job,” Whitford explains. “Individual weed species also react differently to herbicides and to different adjuvants. That’s what can also make spraying difficult—a truly large amount of variability.”
A sprayer pass might decimate one weed species, while another weed species escapes relatively unharmed when the water and accompanying adjuvants are not within specification.
Focus on what chemicals perform best against targeted weed species, Whitford recommends. Performance should then be matched against per-acre costs, revealing what works best and economically makes sense.
“We always want to buy the Cadillac,” Whitford says. “However, the real Cadillac is the top-performing pesticide with a reasonable per-acre cost.”
Problems can originate from multiple sources, so growers have to maintain a tight vigil when spraying.
“In the end, it’s not about the price of the equipment,” Whitford warns. “It’s about what comes out of the nozzle.”