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Figure 3. A tractor and boom sprayer is ideal for large food plots, but since most food plots are small in acreage, they can often be effectively sprayed with a 25-gallon ATV sprayer or even a 4-gallon backpack sprayer.

Sprayer calibration is an important step when applying pesticides and should not be overlooked. Lots of different sprayers can be used in agriculture including some common types such as boomless applicators, air blast applicators, and hand gun applicators. This article will be discussing the process of calibrating a boom sprayer.

Determining Pesticides Per Acre

Boom sprayers are used to make broadcast applications of pesticides to large areas such as fields. Read the label before applying pesticides. Extension professionals actually recommend to read the label before purchasing pesticides. The pesticide label is the law, and the applicator should follow label directions. The pesticide label will tell you how much of the pesticide to apply per acre. The label will also tell the gallons of water needed per acre as recommended by the manufacturer. It is common for a pesticide label to recommend a range of 10 to 40 gallons of water per acre. For boom sprayers, operating pressure, nozzle size and spacing, and operating speed determine the volume of water applied per acre. So how much pesticide do you put in a tank? First we need to determine how many gallons of water you are applying per acre, then we can calculate how much pesticide to add to the tank. In this example we will use the 1/128 acre method, but other methods could be used. Even though we calibrate a sprayer with only water in the tank, personal protective equipment should be worn while calibrating.

Sprayer Maintenance

First, check and clean the tips, filter, hoses, and pump and make sure the sprayer is functioning properly. Put only water in the tank and check the output volume at all the nozzles. This can be accomplished by collecting the volume from each nozzle for the same period of time. The volume may not be exact but they should be within 10 percent of one another. Over time, tips can wear and may need to be replaced.

Steps to Determine Boom Sprayer Calibration

The next step is to determine the length of the calibration course. For the 1/128 acre method we are trying to determine how many feet it takes for one spray nozzle to cover 340 square feet so we will need to measure the distance between nozzles on the boom. For example, nozzles spaced 12 inches apart will require the calibration course to be 340 feet long. An 18-inch nozzle spacing will require a calibration course 227 feet long. A 20-inch nozzle spacing needs a course 204 feet long. To determine the length of the calibration course, divide 340 by the spacing expressed in feet. For nozzles spaced 19 inches apart, or 1.58 feet (19/12=1.58), divide 340 by 1.58 to get the calibration course length of 215 feet.

The calibration course should be measured in the field where spraying will be done. Place a marker of some kind on each end of the calibration course. Then determine the amount of time required to drive the tractor the length of the calibration course. You may drive the course several times in order to calculate an average speed. Keep note of the gear and revolutions per minute (RPM)at which you are operating while driving the calibration course. You should always use the same tractor with the same sprayer. If you change tractors or sprayers you will need to recalibrate.

While the sprayer is stationary, collect water from one nozzle for the length of time that it took to drive the calibration course. The tractor should be operating at the same RPM as it was while driving the calibration course. Measure the volume of water caught in ounces. The number of ounces equals the gallons of water that will be applied per acre. An applicator can change either the speed, nozzle size, or pressure to achieve the desired volume per acre. If an applicator is applying 18 gallons of water per acre and you would like to apply 20 gallons, the simplest thing to change is usually the pressure. Once the pressure is adjusted, the person doing the calibration can collect water from a nozzle for the amount of time it took to drive the calibration course. It may take a few adjustments to get to the desired volume of 20 gallons per acre. If a person has a 100 gallon tank and is applying 20 gallons of water per acre, then the applicator can apply pesticides to 5 acres with every tank full. If the label recommends one pint of pesticide per acre then the applicator will add five pints of pesticide with every full tank. Remember to read the label before purchasing pesticides and give the local Extension office a call to help answer any questions.