Randall B. Heiligmann
Eric Norland
Erdal Ozkan
Good tree seedling survival and growth usually requires control of competing vegetation (weeds) at planting time and for several years thereafter. Herbicides are often the most economical and effective method to accomplish this. Many herbicides used in forestry are applied with a sprayer and their accurate application at prescribed rates requires sprayer calibration. If the sprayer is not accurately calibrated, too little or too much herbicide may be applied, resulting in unsatisfactory weed control, or damage or death of the seedlings.
In Ohio, herbicides applied to young tree seedlings are usually applied in bands along tree rows or in circles around the base of the trees, rather than over the entire ground area (Figure 1). Only a fraction of each acre is actually sprayed. Many different configurations and types of tractor-drawn sprayers can be used to spray herbicides in bands including fixed boom with one or more nozzles, hose and spray gun operated by a person walking, and hose with small hand-held booms.
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| Figure 1. Band spraying tree rows with tractor-drawn boom sprayer. |
Herbicide application rates are generally prescribed in pounds or quarts per acre of ground on which the herbicide is actually applied. A recommendation might, for example, be 3 lbs/A or 1/2 qt/A. Since herbicides are usually combined with water and applied in dilute form, it is necessary to accurately determine the number of gallons of liquid being applied per acre. This enables the sprayer operator to determine the application rate of the sprayer, in gallons per acre, and the ratio at which herbicides and water must be combined to give the desired rate of application, whether the area to be sprayed is 20 acres or 1/2 acre.
This fact sheet describes the steps and considerations for calibrating tractor-drawn boom sprayers. Calibration of hand sprayers is discussed in Natural Resources Fact Sheet F-20; determining the amount of herbicide required to spray a specific tree planting is discussed in Natural Resources Fact Sheet F-23; and calibrating boom sprayers for agricultural crops is discussed in Extension Bulletin AEX-520.
To assure accuracy in calibration, take all measurements carefully and carry all calculations to at least three decimal places where appropriate. Final measurement of herbicide volume or weight should be done as accurately as equipment will allow.
Follow these steps to determine the sprayer application rate:
Single Nozzle Per Band- With the tractor stationary, and the sprayer operating at the same pressure that will be used in the field, place a jar with a capacity of at least one quart under each nozzle and measure the number of seconds needed to spray one quart from each nozzle (Figure 2a). The flow rate in gallons per minute (GPM) for each nozzle is then determined as follows:
GPM = 15
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number seconds needed to spray 1 quart
Example: With the sprayer stationary and operating at the pressure to be used in the field, it takes 60 seconds for a nozzle to spray 1 quart. The delivery rate of the nozzle is:
GPM = 15/60 = 0.25 gallons per minute
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| Figure 2a. Boom with single nozzle spraying each row of trees. |
Multiple Nozzles Per Band (Overlapping Spray Pattern)- If multiple nozzles are used to spray a single row (Figure 2b), determine the delivery rate of each nozzle for the row and add the amounts together to obtain the total amount of spray being delivered to the row.
Example: The spray boom to be used is configured so that two nozzles with overlapping patterns are used to spray a single row. The flow rate of one nozzle is 0.25 GPM, and the other is 0.29 GPM. The total volume of spray delivered to the band is 0.54 GPM.
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| Figure 2b. Boom with overlapping nozzles spraying each row of trees. |
Calculate the average flow rate in gallons per minute for the nozzles on the boom. If the flow rate of any nozzle is more than 10 percent higher or lower than the average flow rate, clean the nozzle and remeasure flow rate. Replace any clean nozzle that differs from the average flow rate by more than 10 percent. Repeat this process until the variation in flow rate for all nozzles is within 10 percent of the average.
Example: The nozzles on a 4-nozzle boom deliver 0.26, 0.27, 0.28, and 0.30 GPM. Average delivery rate is 0.2775 GPM. The difference between the average flow rate and individual flow rates for all nozzles is within 10% of the average delivery rate. However, if the delivery rates were 0.26, 0.27, and 0.28, and 0.32 GPM, average delivery rate would be 0.2825 GPM, and the nozzle with the 0.32 GPM delivery rate would have to be replaced.
Measure a known distance, in feet, and time the tractor over that distance. Operate the tractor at the same miles per hour (MPH) and revolutions per minute (RPM) to be used in the actual spraying. Begin the test run from a standing start far enough ahead of the measured course so that the tractor is traveling at the desired speed before coming to the start of the course. The speed of the tractor is calculated as follows:
(distance traveled in feet) x (0.68)
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MPH = number of seconds needed to travel
pre-measured distance
Example: If it takes 29 seconds to
travel 150 feet, the travel speed is:
MPH = (150 feet) x (0.68) = 3.5 MPH
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29 seconds
The acreage application rate, gallons per acre (GPA), of the spray boom is determined as follows:
GPA = (gallons per minute) x (5940)
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(MPH) x (band width in inches)
"Gallons per minute" is the boom average for either single or overlapping nozzles. Note that band width is expressed in inches.
Example: Using a boom with an average nozzle spray rate of 0.25 GPM, a speed of 3.5 MPH, and spraying a 24-inch wide band, the gallons per acre application rate is as follows:
GPA = (0.25 GPM) x (5940) = 17.68 GPA
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(3.5 MPH) x (24 inches)
Compare the calculated application rate with that recommended for the specific herbicide to be used (refer to label on herbicide container). If the calculated rate is more than 5 percent higher or lower than the recommended rate, small adjustments in the sprayer speed and/or pressure can be made. Do not exceed the pressure recommended for the nozzles. If reasonable speed and/or pressure changes will not achieve the desired application rate, a different set of nozzles may be needed.
The amount of herbicide and water needed to spray a planting depends on the sprayer calibration,the recommended herbicide rate, and the number of acres to be treated. Follow these steps to determine the amount of herbicide and water required for a specific planting:
acres sprayed = tank capacity in gallons
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GPA
Keep in mind that "acres sprayed" is the actual amount of ground that is sprayed. When spraying bands, this is not the same as the number of acres driven over.
Example: Using the example above, if the spray tank held 100 gallons of water, the calculation would be as follows:
acres = 100 gallons = 5.66 acres of actual ground
sprayed ___________ sprayed with one tankful
17.68 GPA
amount of recommended number of herbicide = rate of application X acres sprayed add to full on herbicide with a full tank sprayer
Example: If the recommended herbicide application rate is 4 lbs/A, you need to add 22.64 pounds of herbicide to a full 100 gallon spray tank.
amount of herbicide to add to full sprayer = (4 lbs/A) x (5.66 A/tankfull) = 22.64 lbs per tank
When spraying herbicide in bands, only a fraction of each acre (the band) is actually being treated. To determine the overall field size that can be band sprayed with a full tank, perform the following calculation:
overall actual acres distance between center
field = sprayed with X of bands in inches
size full tank ________________________
width of bands in inches
Example: You plan to spray 24-inch bands whose centers are 84 inches (7 feet) apart (Figure 3). Having previously determined that you can spray 5.66 acres of actual ground with a full 100 gallon tank, the overall field size that can be band sprayed is:
overall field size = 5.66 acres X 84 inches = 19.8 acres
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24 inches
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| Figure 3. In this example, 5.66 acres of actual ground have been sprayed. |
A two-step calculation is required to determine the amount of water and herbicide needed to spray smaller fields that do not require a full tank.
Step A: Determine what proportion the size of the actual field to be sprayed is of the field that can be sprayed with a full tank.
Example: If we wished to band spray a 15-acre field using the 100-gallon sprayer described above which will spray 19.8 acres when full, the proportion is calculated as follows:
15 acres = 0.757 or 75.7% ________ 19.8 acres
Step B: Reduce both the amount of water and the amount of herbicide to the proportion calculated in Step A above.
Example: The amounts of water and herbicide needed to band spray the 15 acre field are:
amount of water needed =(100 gal) x (0.757) = 75.7 gal
amount of herbicide needed = (22.64 lbs) x (0.757) = 17.1 lbs
When adding herbicides to a sprayer, it is convenient to use either a container that holds exactly the desired amount of herbicide, determine the number of teaspoons or tablespoons equal to the desired amount, or mark on a larger container exactly how full it should be to contain the desired amount of herbicide. When measuring herbicides initially, be sure to use liquid measurements for liquid materials (e.g., liquid ounces in a measuring cup) and dry measurements for powders and other dry formulations (e.g., weight in ounces). Use Table 1 to determine equivalent measures and conversions.
Spraying Pressure--Spraying should be done at the same tank pressure as when calibrating. Spraying at a higher or lower pressure will result in more or less herbicide being applied as the sprayer passes over the ground.
Nozzle Type And Size--Generally, even flat fan spray nozzles are most desirable for band or spot spraying. They provide the most even distribution of spray material over the area treated. Sprayers must be calibrated for each nozzle type and size. The larger the nozzle, the more spray is put out as the sprayer passes over the ground. Thus, to maintain a particular rate of application when larger nozzles are used, smaller amounts of herbicide will be added to the water.
Speed Of Travel--Spraying should be done at a uniform speed and at the speed used when the sprayer was calibrated. Slower or faster speeds will result in higher or lower rates of herbicide application.
Nozzle Alignment--Make sure nozzles are properly aligned to applyspray uniformly where desired. If nozzles require overlapping, check the nozzle catalog or instructions to determine the overlap required for specific nozzles.
Inspect And Clean Nozzles Periodically--A common cause of non-uniform coverage is clogged nozzles. Watch the nozzles periodically while spraying to detect clogging. Always carry tools for cleaning nozzles and extra nozzles. Replace defective nozzles immediately.
| Table 1. Equivalents and conversions. |
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| DRY MEASURES |
| 1 pound = 16 ounces |
| 1 pound = 454 grams |
| 1 ounce = 28,35 grams |
| 1 gram = 1000 milligrams |
| LIQUID MEASURES |
| 1 gallon = 4 quarts |
| 1 quart = 2 pints |
| 1 pint = 2 cups |
| 1 cup = 8 fluid ounces |
| 1 fluid counce = 2 tablespoons |
| 1 tablespoon = 2 teaspoons |
| 1gallon = 3.785 liters |
| 1 liter = .264 gallon |
| 1 teaspoon = 5 milliliters |
| 1 tablespoon = 15 milliliters |
| 1 fluid ounce = 29.56 milliliters |
| 1 cup = 236 milliliters |
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-1868