Expert insights into achieving good deposition. By Anna Mouton.
Effective crop protection is conditional on effective spraying. Growers need to apply the appropriate products at strategic times using correct techniques. Sounds straightforward — except spray application turns out to be a complicated matter.
Successful crop protection requires uniform spray deposition over the whole tree. But trees are three-dimensional objects with irregular shapes and uneven densities. And tree canopies change with the shifting seasons and from nursery to maturation. How can growers accommodate all this variability within the constraints of day-to-day farm management?
Fresh Quarterly turned to Dr Gideon van Zyl and Philip Rebel from specialist agricultural consultancy ProCrop for their views on how growers can optimise spray applications. The first step is understanding good deposition.
What is good deposition?
Good deposition starts with correct quantity. The amount of chemical deposited has to be sufficient to counter the intended pest or disease. However quantity isn’t everything. “The plant-protection product needs to come into contact with the pest or disease,” says Van Zyl. “We don’t know where an infection will occur, so we need an even distribution of the plant-protection product on the surface that we want to protect.”
The distribution of a chemical on a single target — such as a leaf or fruit — constitutes deposition quality. A small number of large droplets may deposit an equal quantity of chemical on a target as a large number of finer droplets. But the large drops will be sparsely distributed and ultimately deliver less chemical to the intended pest or disease. Finer drops that are evenly distributed are more likely to make contact and transfer an effective dose of pesticide or fungicide.
Van Zyl explains that the majority of chemicals used for crop protection do not translocate throughout the plant. At most a chemical will translocate within a leaf or from a leaf to the nearest growing point depending on mode of action. In practice this means that poor deposition quality in any part of the tree renders it vulnerable. For this reason uniformity of deposition quality is the third prerequisite for good deposition.
Does this imply that good deposition requires drenching the tree in chemicals? Usually not. Too much liquid applied to a target will lead to merging of droplets and runoff — ultimately reducing deposition quality and quantity. “A spray application with smaller droplets increases the chances of better coverage and is definitely better if you’re spraying a contact agent preventively during the season,” affirms Rebel.
The right machine for the job
Using the correct spray machine is essential for good deposition quality and uniformity. “Your machine must match your tree, or your tree must match your machine,” says Van Zyl. He points out that low-profile machines that were designed for open-centre canopies are not a good fit for tall trees because the distance between the machine and the target is variable. Drops aimed toward the top of the canopy tend to have insufficient energy to reach their target and some nozzles may direct the spray above the tree.
Van Zyl stresses that increasing the air volume will not fix the problem because drops aimed at the lower parts of the canopy end up travelling too fast. These drops either bounce off foliage or speed straight through the tree.
The solution is to use high-profile machines for taller trees because they direct horizontal airflow from a tower that distributes the spray evenly across the canopy.
Van Zyl suggests that growers evaluate the direction of the air generated by their machines by the simple method of tying streamers — for example pieces of hazard tape — to the outlets. The streamers will show where air is going and help to identify nozzles that could be removed.
Conform to your canopy
Canopy-adapted spraying lets the characteristics of the canopy guide the application method. “You adjust variables like forward speed and air volume to better suit the size and shape of a tree canopy, instead of using constant spray-machine settings for every orchard on a farm,” says Rebel.
Imagine a tractor driving past a single tree in the orchard. The volume of droplet-containing air directed at this tree is a function of the forward speed of the tractor and the air delivered by the machine. Increase tractor speed and the volume per tree is reduced because the window of time during which droplet-containing air is delivered to each tree is smaller.
Van Zyl points out that most pumps and sprayers are designed to run at a power take-off speed of 540 rpm — revolutions per minute — so this is the most common setting at which to power fans of spray machines. But modern canopies are lower in volume and less dense than conventional orchards. Using conventional high air-volume spray machines at too-low forward speeds will result in spraying through these trees. Forward speeds that are too fast combined with too-low air volumes will fail to penetrate the canopy.
“You want your system to be in equilibrium,” explains Van Zyl. “You first need to adjust your fan-blade pitch, second, your forward speed, and third, your fan speed so that the air column carrying droplets just penetrates the canopy but without spraying through the tree.” Spray that travels through the tree without landing makes no contribution to pest or disease control.
Streamers are again a useful tool to determine whether a combination of fan-blade pitch, forward speed and fan speed is optimal. Tie the streamers to the tree on the side opposite to the spray machine and observe their movement when the tractor passes. Estimate the angle of the streamer to the vertical. An angle of 25°–45° is perfect. An angle of less than 25° indicates that the air-momentum combination is too low and an angle of more than 45° tells you that the air-momentum combination is too high — you are probably spraying right through the canopy.
Make short work of spraying
The starting point of planning spray applications should be determining the maximum safe tractor speed for a given orchard. “Faster is always better,” asserts Van Zyl. “Why? Because we need to get through our spraying!”
Van Zyl explains that being able to spray a farm quickly is crucial for effective disease control. He cites the example of apple scab where full spray coverage must be obtained within 2.5 days leading up to high-risk periods. Any earlier than 2.5 days and the tree will have produced new leaves that are susceptible to infection. “It is critical to spray as near as possible to the infection period so as to ensure that the percentage of the tree that is protected is as high as possible.”
Adopting lower-volume and canopy-adapted spraying will also contribute to faster spraying. “The estimated global average for spray time per hectare is 33 minutes for apples,” says Van Zyl. “In South Africa you still have people who take longer because they spray volumes that are too high based on the tree-row-volume dosing model.”
Water is simply a carrier for chemicals. Good coverage is possible when spraying as little as 350 litres per hectare in modern orchards. Keep in mind that the concentration of the chemical needs to increase when the water volume is decreased so as to deliver the correct dose to the tree.
Spraying lower volumes not only saves time but also improves deposition quality while reducing the risk of runoff.
Orchards of the present and future
Both Van Zyl and Rebel emphasise that modern high-density plantings lend themselves better to successful spray applications. “Our newer trees have a much smaller volume and are more uniform with less complexity on the outside,” says Van Zyl. Combine this with a flat orchard floor and you have the ideal conditions for rapid and effective spraying.
Rebel agrees that the orchard of the future will simplify spray applications. “In a situation where a farm only has tall spindle-shaped trees it would be possible to use a single nozzle layout on the entire farm. All you’d need to do is change your tractor speed and power take-off to adapt your air volume for each orchard.”
Deposition in traditional orchards with large trees is more complex. “You need more droplets to cover all the surfaces and you’ll have more losses during application,” says Van Zyl. But there are steps that growers can take to improve their chances of success.
Remove branches that obstruct spraying. Modern pruning techniques help to open the canopy and allow for better spray penetration. “There is this idea that you need to spray greater water volumes to get into a dense canopy. That doesn’t work,” comments Van Zyl. “Spraying more water at a wall isn’t going to let you spray through it.”
Van Zyl also recommends maintaining an even orchard floor as an uneven surface can cause the spray machine to tip and lead to variation in deposition. This illustrates how the design specifications of the orchard of the future extend further than just the training systems for the trees — even conditions for tractors need to be considered.
Optimal spray applications depend on the interaction of multiple factors. Van Zyl routinely presents an entire course covering spray application on deciduous fruit whereas this article barely scratches the surface of the topic. But Rebel is confident that the benefits of more efficient spraying are within reach of all growers. “If you are prepared to invest a little more time in the management of spray applications, you can be more effective and make cost savings without having to make a lot of changes or change your equipment.”
Bonus: Correct calibration in nine steps
- Calculate the tree-row-volume for dose adjustment for the orchard in question.
- Depending on the type of spray machine used and the size of the canopy, adjust the fan-blade pitch to produce the desired air volume.
- Calculate the allowable forward tractor speed.
- Bring the combination of tractor and sprayer into the orchard to test forward speed. Starting at 540 rpm, determine the fastest tractor speed that is not dangerous and allows for full canopy penetration at the top, middle and bottom of the canopy.
- If less air volume is required after pitch adjustment, manipulate the power take-off speed downwards until the air column matches the canopy, taking reduced speed into account. Make sure the sprayer pump and tractor clutch are designed to work at lower power take-off speeds.
- Decide on a spray application volume per hectare applicable to the target orchard.
- Calculate the litres per minute required for both sides at the selected application volume.
- Divide the litres per minute by the number of nozzles. Select the correct nozzle-pressure combination with consideration of drop size from nozzle charts provided by the nozzle manufacturer.
- Measure the actual delivery for at least 60 seconds. Make the necessary pressure adjustments to change the application volume if there is a discrepancy.