There is no perfect day to spray, plain and simple. Growers in most years are often backed in a corner, avoiding wind one day, only to face rain the next. Delays in spraying means the weeds grow larger and more difficult to control with herbicides.
John Byrd, weed scientist, Mississippi State University, recommends growers make certain that all tips and screens are appropriate and uniform across the width of the boom – particularly on equipment that is new to a given operation. Regardless of sprayer brand, uniformity in tip size and screens is necessary to properly filter particles. Variations in pressure can directly affect flow rate through a tip.
Byrd reminds growers to consider a practical application volume. “What maximizes the land area you can cover? You certainly want to produce a relatively small droplet, but not small enough to increase the potential for off-target movement. I know a lot of people want to drop below 10 gallons per acre to maximize ground covered. Personally, I’m a bigger fan of a minimum 12 to 15 gallons 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, and environmental conditions such as wind speed and direction have to be closely monitored. Byrd advocates for a medium droplet size that allows for good coverage, but reduces drift risk. Typically, 01, 02, and 03 nozzles provide that optimal size.
Most labels recommend a spraying window of 2 mph to 10 mph – a relatively narrow spectrum. “Wind is a big issue for growers and 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 a temperature inversion as air away from the surface gets warmer. When inversion occurs, spray has increased 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 growers put on a field during spraying. Director Fred Whitford, Purdue Pesticide Program, believes water is the foundation of a successful spray pass. “Water is not on the sideline watching the game, it’s one of the players,” Whitford emphasizes.
Sometimes pesticides don’t control weeds like they should – and one of the reasons is tied to the quality of the mix water. Water may have a high mineral content such as calcium and magnesium that binds with a herbicide product that actually dilutes the rate being applied. In other cases, acid or alkaline (called pH) out of balance can lead to chemical breakdown and less going into solution. On other occasions, growers may fail to use a surfactant that the label requires 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 that 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 may decimate a particular weed species, and yet another weed species escapes relatively unharmed when the water and accompanying adjuvants are not within specification.
Buying a Cadillac
Whitford recommends growers focus on what chemicals perform best against targeted weed species. Top performance should then be matched against costs per acre – revealing what works best at the most economical cost. “We always want to buy the Cadillac. However, a pesticide that is a top performer at a reasonable cost per acre is the real Cadillac.”
Problems originate from multiple sources and growers have to maintain a tight vigil when spraying. “In the end, it’s not about the price of the equipment you’re using,” Whitford warns. “It’s about what comes out of the nozzle.”