© Barbara Helgason | Dreamstime.com
Orchard in Osoyoos
Hitting the desired locations with sprayed product in a dense canopy may feel like part math equation and part crap shoot. Too much spray means waste. Too little means not enough product hits the targets for the desired effect.
There’s more to the conundrum than just volume or pressure and Tom Wolf of Agrimetrix presented his case for why air blast is the way to go – especially when dealing with dense canopies – at the Pacific Agriculture Show in Abbotsford in late January.
“We experimented with air blast in the 80s and 90s but it didn’t seem to catch on,” he says. “We still believe in air blast.”
Wolf explained that development in spraying technologies has been driven by spray drift because, “we can’t afford to have the product go where it doesn’t belong.”
Droplet size and how those droplets work in the environment during the time of spraying is the key and this is where the mix of math and crap shoot come in.
“All nozzles produce droplets of a large variety of sizes,” Wolf says. “They are at the mercy of ambient atmospheric conditions.”
By the time droplets get 50 centimeters below the canopy, they slow down. Add to this the fact that moving a nozzle forward (such as being propelled on a tractor) moves the droplet both forward and down. Adjusting nozzles to spray slightly backwards help to compensate for the forward motion.
“They will hit something vertical on their way into the canopy,” he says. “This is why boom height is so essential.”
Large drops easily penetrate the ambient air movement to hit the leaf. Where air assist comes in is helping smaller drops also hit the leaf instead of going around it. This is essential with small leafed canopies.
“Small targets and small drops get along extremely well,” notes Wolf.
The same relationships works for large drops and large targets. When dealing with a cascading canopy, smaller drops are more effective, but don’t think increased pressure is the solution. Wolf explains the increased pressure makes the spray finer and increases the volume, but doesn’t have the ability to penetrate the canopy as needed.
In lab tests, Wolf used targets and a dyed solution to simulate a spray pass. Experiments were done with air blasts versus hand booms and in all cases, the air blast achieved higher coverage of canopy foliage by a significant margin. Apples, blueberries and grapes were all tested in the experiment.
Fortunately, Wolf gave a step by step process of achieving better canopy spray coverage.
First and foremost he cautions that this is an ever-evolving process throughout the season.
“If you go full volume the whole season you’d be over-dosing,” he says.
Take the sprayer into the canopy half-full of water. Ensure the weather and time of day is similar to the conditions you’ll be spraying in. Calibrate sprayer to the crop based on ambient conditions.
Later in the season, as foliage increases, a higher volume will be needed.
Put flagging tape on the canopy to see how it moves in the air blast.
“You want some air to move through that canopy,” he explains. “You want to ruffle the leaves… so that spray gets through. It takes a few minutes to set that up”
Place water-sensitive paper throughout the canopy to see the coverage after spraying the water.
“You have to get the spray onto the target,” notes Wolf. “The return on further dosing is very diminishing.”
Make adjustments to nozzle and air blast depending on what you see in terms of tape movement and water coverage on paper.
Mix spray accordingly and spray using the adjusted settings.
Get the most out of sprays by calibrating and testing spray settings in the right conditions and adjust throughout the season. Just as plant growth constantly changes, so should the plan for spraying to reach the targets within the plant canopy.