Feb 10, 2017
Crop-adapted spraying pays dividends

Crop-adapted spraying (CAS) is a method for apple orchardists to match their airblast sprays to the size, shape and density of their orchard blocks.

Jason Deveau, application technology specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), focused on spray innovations at the recent Great Lakes Fruit, Vegetable & Farm Market EXPO in Grand Rapids, Michigan. He said that when CAS is achieved with sufficient accuracy, pesticide efficacy is maintained and waste is greatly reduced.

Based on international models for dose adjustment, and intended to be simple and intuitive, the CAS method was tested in three Ontario orchards for three years, and independently in a Nova Scotia orchard for two, Deveau said.

The CAS model proposes an airblast nozzling solution that dictates spray volume per acre (or hectare). The operator must still conduct a few qualitative tests to ensure the air settings, travel speed and resultant spray coverage are appropriate for the weather conditions.

“The results of our work indicate that a sprayer calibrated using the CAS method provides the same control as one calibrated using conventional methods, but CAS can save an average 20 percent of the season’s spray,” Deveau said. “The pesticide savings is an appealing benefit, but the primary objective of CAS is to account for variability in orchard blocks and achieve consistent coverage throughout an orchard operation.”

The CAS method is accessible through a smartphone app called OrchardMAX, freely available for Apple and Android systems. OrchardMAX can be downloaded here: http://sprayers101.com/orchardmax.

Deveau outlined a scenario in which an orchardist following an IPM protocol identifies a pest that poses an economic threat.

“This is no surprise because it’s an annual pest, and spraying is only a matter of when, not if,” he said.

The operation is 150 acres and runs three airblast sprayers; two have a tower and one does not. Multiple varieties are planted in several blocks on different rootstocks and they are at different stages of maturity. The newer blocks are trellised high-density trees and the older blocks are semi-dwarfs on different row spacings. Some trees are not yet pruned.

“Fortunately, the orchardist has the pesticide label to guide them,” he said. “It helpfully prescribes a range of doses per planted area, depending on the pest pressure. It advises that the orchardist mix an absolute amount of product in enough water to ensure the tree is sprayed ‘to the point of runoff,’ ostensibly assuming that if the tree is saturated, it will have received the ‘correct dose.’”

Deveau said the applicator would recognize this label is vague, and would elect to rely on what has worked historically: A water-soluble pouch is dropped into each tank (close enough), and each sprayer operator is instructed to drive at an efficient speed (get it done because rain is coming), spraying until the tank is empty (which means they must have hit the target). If a tank is running low before the job is done, speed up and stretch it. If the spray is overshooting a younger planting, turn off the top nozzles.

“It’s a fanciful scenario, but I’ll bet you recognize your own operation in there somewhere. The question is: Is there a problem with spraying this way if it results in a respectable crop of quality apples?”

Deveau said agricultural engineers specializing in application technology in Spain, Australia, Great Britain and the United States say there is a problem, “and on behalf of Canada, I completely agree with them.”

Orchard, sprayer variability

“The fundamental problem is inconsistent spray coverage and excessive waste (of time, water and pesticide) due to variability,” Deveau said. “Our scenario notes multiple sprayer operators, different models of sprayer, and a range of morphologies, orchard layouts and canopy management practices.”

Deveau said international, peer- reviewed articles stretching back to the ’60s have conclusively demonstrated order-of-magnitude differences in the area-density of orchard canopies from one acre to the next.

“There can even be fold-differences in canopy area-density in the same planting as the season progresses,” he said. “A label prescribing a fixed dose based on the area planted is not appropriate for any vine, bush or tree crop, and the result is that more crops are over- or under-sprayed than receive appropriate coverage.

“Let us not forget the variability that comes from an uncalibrated or poorly adjusted sprayer,” he said.

Beyond the immediate impact on efficiency, variability makes it difficult to diagnose pesticide effectiveness, Deveau said.

He cited a scab outbreak in Ontario a few years ago that elicited questions about timing, weather, product choice and resistance.

“There was very little attention given to spray coverage, which in my mind should have been the first question, if only to eliminate it as a potential culprit,” he said. “This is because each operation interprets labels differently, and very few confirm coverage in any quantifiable way. This practice makes it difficult to identify a cause when crop protection fails.”

The maintenance, calibration and operation of an airblast sprayer is an involved process. Photos: Gary Pullano

Optimizing pesticide rates

Deveau said what is needed is a way to adjust the amount of pesticide per unit ground area (i.e. the label’s prescription) to achieve consistent foliar coverage for canopies of varying shape and density.

He said the CAS method combines aspects of many previous models used to determine pesticide efficiency.

“It is neither complicated nor sophisticated,” he said. “It formalizes a series of qualitative calibration techniques and the objective is to achieve a target foliar coverage pattern. When achieved with sufficient accuracy, pesticide efficacy is maintained and waste is greatly reduced.”

He said CAS relies on a few critical assumptions.

“The first assumption is that the sprayer operator’s typical ratio of formulated product to carrier is appropriate. We need a starting point for adjusting the amount of pesticide per unit area, and typically the label is too subjective. Further, the operator might be tank mixing other products and electing to reduce the dose of each tank mix partner, or adjusting the ratio of product to carrier to reflect pest pressure. The appropriateness of this assumption is evidenced by a history of satisfactory pest control in the orchard.

“The second assumption lies in defining a threshold for sufficient coverage, and this is a real challenge,” Deveau said.

Applications can be concentrate or dilute. Some products are locally systemic while others have limited redistribution. Even the droplet size employed will have bearing.

“So, how does one draw a universal line in the sand and say, ‘this much is enough?’”

Deveau said a literature review, combined with practical experience, “has led us to propose 10 percent to 15 percent coverage comprised of a minimum of 85 medium-sized droplets per square centimeter as a reference point for satisfactory foliar coverage.”

The figure is intended to be practical and versatile to safely represent sufficient coverage for most foliar insecticides and fungicides, he said. It is not intended to be a “rigorous, scientific absolute.”

“For example, a drench application such as streptomycin or dormant oils, will obviously require more coverage,” he said. “Plant growth modifiers like thinners, stop-drops and foliar nutrients may or may not, and have their own unique criteria.”

The maintenance, calibration and operation of an airblast sprayer is an involved process, Deveau said. Collectively, the sprayer setup, weather and crop morphology all influence the coverage obtained from an application. A fundamental understanding of application technology is required before attempting to optimize dosage using the CAS method.

Gary Pullano, associate editor

Follow these CAS steps

Step 1. The sprayer should receive all seasonal maintenance prior to first use and undergo a visual inspection before each spray day.

Step 2. Park the sprayer in an alley between rows of trees and tie 25 centimeter (10 inch) lengths of ribbon to the ends of the deflectors (if present) and the nozzle bodies. Turn on the air and extrapolate where the nozzles and deflectors are aimed. Adjust deflectors and turn off nozzles that will spray over or under the tree canopy. Consider using air-induction hollow cones in the top positions of each boom to reduce drift. You may have to increase the rate in those positions to compensate for the fact that nozzles producing larger droplets produce fewer droplets.

Step 3. Confirm ground speed with a half-full sprayer in the orchard using GPS or a calibration formula.

Step 4. Affix 25 centimeter (10 inch) ribbons to far side of three trees. Tie them at the top and at the widest portions of the canopies. Drive past in the spraying gear at the ideal RPM with the air on, and ensure the ribbons waft outwards. This will determine if more/less air is required from the airblast sprayer, and if the operator should speed up or slow down during spraying. This is also an opportunity to perform gear-up-throttle-down if the sprayer is using a positive displacement pump.

Step 5. Place water- sensitive paper at the top, center and bottom of the tree canopy and spray water. As an approximation, if coverage exceeds 10 percent to 15 percent surface-area and 85 discrete droplets per square centimeter, reduce output in those positions by replacing nozzles with lower outputs.

If less than ideal coverage is achieved, increase the nozzle rates in those positions. Excessive coverage may be unavoidable in the outer edge of the canopy, given that spray must pass through to get to the center. Be aware that ambient wind speed and humidity have significant impacts on coverage. Therefore, only test coverage in conditions similar to your typical spraying conditions.

Step 6. When the canopy grows and fills in sufficiently, usually after petal fall, repeat steps four and five (this is typical for semi-dwarf, but likely isn’t required for young and/or high-density plantings).

“If you are suspicious that the spray is being stretched too thin or you are unsatisfied with the coverage, you may have to increase the output,” said Jason Deveau, a technology specialist in Ontario.

This is more of an issue with larger trees. Early in the season, wind travels relatively unimpeded in a high-density orchard and will blow the spray off course, reducing coverage and requiring higher water volumes or possibly more air to compensate. As the trees fill in, the average wind speed is reduced and more spray can impact on the target. Therefore, increasing spray volume after petal fall may not be required in a high-density orchard.

When the correct sprayer settings and volumes have been determined, the operator will mix their spray tank as they would for their typical application. The sprayer will likely cover more orchard than it has in the past, and the operator will have to reassess how many tanks are required pre and post petal-fall. It is not advisable to go below 400 L/ha (~40 gpa).

OrchardMAX (the CAS calculator app) helps the operator choose the correct rates for each nozzle position.

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