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Spraying Guidelines

Sample Calculations

Airblast Sprayer Calibrations

Jim Dutcher Entomology Department University of Georgia – Tifton Campus Coastal Plain Experiment Station Tifton, GA 31793

The airblast sprayer is one of the more important pieces of equipment in the pecan orchard and the winter is a good time for sprayer calibration. These machines are run continually during the season and they have to be set up to deliver a uniform spray that covers all the foliage and fruit in a precise volume per acre. Pecan growers typically use between 75 and 150 gallons of spray solution per acre depending on the size of the pecan trees, the space between the trees, and the type of pest that is the target of the treatment. There is an optimum spray volume for each situation and a spray that wets the entire surface of the leaves and nuts with very little runoff or dripping onto the ground is ideal. The calculations are fairly straightforward with easily measure parameters and two variables that have to be estimated based on the situation. First, let’s consider ground speed. This should be 2 mph or less for most applications. Two mph is ~ 2.9 feet per second. Driving a spray rig past both sides of a tree with a 60 ft canopy width will take 38 seconds, and a smaller tree with a 40 ft canopy width will take 27 seconds. If the sprayer pump and fan are powered by the PTO then the driver has to find the best forward gear to achieve both a speed close to 2 mph at the engine speed that delivers sufficient power through the PTO to blast the air through to the top of the tree. If the sprayer has its own power then the tractor engine speed and gearing can be adjusted for the best fuel economy with sufficient power to haul the 2-4 tons of water in the full tank plus the weight of the sprayer. Second, let’s consider the spray volume. To deliver 100 gal per acre to 12.1 trees/acre with a 60 ft canopy width all the nozzles together have to deliver 8.3 gallons in 38 seconds or 13.5 gallons per minute. To deliver 100 gal per acre to the 27.2 trees/acre with a 40 ft canopy width all the nozzles together have to deliver 3.7 gallons in 27 seconds or 8.2 gallons per minute. Two methods are used to estimate the amount of spray volume required to cover the tree canopy. Spray to runoff is the easier method but lacks accuracy. The trees are sprayed in a trial with a range of increasing volumes (e.g. 75, 100, 125, 150 gal per acre) during the growing season when the trees have leaves and fruit. When the volume increases to the point that solution starts dripping off the foliage then this is the point of runoff indicating that the canopy cannot hold any more spray solution. The spray volume is volume just below the point of runoff. The accuracy depends on how close the intervals are between spray volumes in the trial. Tree row volume is the more accurate method but requires some additional measurements and calculations. In this method, the actual volume of the tree canopy and the amount of spray solution required to fill the canopy with spray (dilute spray volume) are estimated and the spray volume is selected based on the dilute spray volume and a concentration factor selected for each type of pest that is targeted by the spray. Third, the output of all nozzles has to be adjusted to deliver the calculated spray volume. The spray volume from each nozzle is measured by simply collecting spray from the each nozzle over a ten second interval and in a flask or jar with the volume marked on the side. For pecan trees where most of the nut production is from the periphery in the top of the canopy, two thirds of the spray is usually deliver to the top one third of the tree canopy and one-third of the spray volume is delivered to the bottom two-thirds of the tree canopy. Fourth, once the volute is set up to deliver the desired spray volume and the selected pump pressure and tractor speed the final adjustments are made in the field with water sensitive cards. The winter months are a good time to test the rig out since there are not leaves in the trees. The cards can be hung in the bare canopy to measure the spray pattern. Poles made from electrical conduit are used to distribute the cards in the tree canopy. Cards are fastened to clips set at 2 yd intervals along the pole. Rather than setting the poles in the ground they are hung in the trees with a hook at the top of the pole. and the tractor is driven past the pole with the sprayer running. Once the spray pattern is measured on the cards, the nozzles can be switched around on the volute to add spray volume in under-sprayed areas and subtract spray volume in over-sprayed areas. Once the sprayer is set up it should stay in adjustment for many hours of application time during the season. The sprayer loses its original adjustment when debris clogs the nozzles and strainers, rubber washers swell and block the holes in the whirl plate, the orifice in the nozzle becomes larger with use over several seasons or the pump wears out and is not able to produce sufficient pressure. The following examples demonstrate the calibration of a PTO powered airblast sprayer with the tree row volume method. First, the ground speed is adjusted to 1.9 mph and the pump pressure is set at 90 psi by selecting 1st gear high range for the tractor and the rotational speed 1700 rpm for the PTO. These are held constant. Second, the tree row volume is calculated from the formulae below. The calculations give the nozzle output and the concentration factor. The following examples demonstrate the calibration of a PTO powered airblast sprayer with the tree row volume method. First, the ground speed is adjusted to 1.9 mph and the pump pressure is set at 90 psi by selecting 1st gear high range for the tractor engine speed of 1700 rpm and the rotational speed 540 rpm for the PTO. These are held constant. Second, the tree row volume is calculated from the formulae below. Two examples are given where the first orchard (Figure 9) has trees 44 feet high and 38 ft wide that are planted on a 40 by 40 ft spacing and the first bearing limbs are 4 feet from the ground and the second orchard (Figure 10) has trees 65 feet high and 57 ft wide that are planted on a 60 by 60 ft spacing and the first bearing limbs are 18 feet from the ground. Pecan growers typically treat orchards in these two sizes with 100 gal of spray per acre and the sprays are effective yet the concentration factors are fairly high (7.7 X to 9.1 X in these two examples) in comparison to other fruit crops (typically 4X to 5X). The pecan canopy, however, bears a high portion of the nut crop on the periphery of the tree and the foliage in the center of the canopy is sparse relative to other fruit and nut trees. In addition, most fruit crops are treated with an ‘effective spray volume’ of 0.7 gal of spray per 1,000 ft3 and this value may be lower for pecan trees. These and other factors may lead to the effectiveness of sprays against most nut pests and some foliage pest. Pests that typically colonize the foliage in the center of the canopy such as black pecan aphids and pecan leaf scorch mites have been primary and recurring problems in pecan trees and these pests may be avoiding treatment with low volumes of highly concentrated sprays. Third, the spray pattern can be matched to the tree canopy shape. Ten trees were measured at each location to represent the overall tree canopy size. The volute has larger nozzles at the top of the volute and smaller nozzles at the bottom and intermediate nozzles in the center. The first orchard has fairly good production of the lower and middle canopy. Nozzles are selected to deliver 40% of the spray to the bottom 60% of the canopy and 60% of the spray to the top 40% of the canopy or ~10 gal of spray per minute from a height of 4 ft to 28 ft and ~14 gal of spray per minute from a height of 28 ft to 44 ft. The second orchard has poor production of the lower and middle canopy and good production at the top of the canopy. Nozzles are selected to deliver 33% of the spray to the bottom 67% of the canopy and 67% of the spray to the top 33% of the canopy or ~12 gal of spray per minute from a height of 18 ft to 40 ft and ~24 gal of spray per minute from a height of 40 ft to 65 ft. Fourth, minor adjustments are made by trial and error while actually spraying with the configured sprayer into the canopy onto strategically-placed water sensitive cards.

Useful Web Sites

Ohio State University Bulletin
West Virginia Experiment Station
North Carolina State Article

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Email: jimdutcher@lycos.com