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 of accomplishing 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.
Herbicide application rates are generally prescribed in pounds or quarts per acre. A recommendation might, for example, be 3 lbs/A or 1/2 qt/A. Sprayer calibration involves determining how much herbicide and water should be mixed in the sprayer to deliver the herbicide at the desired rate, whether the area sprayed is 1 acre or 1/1000 acre.
This fact sheet describes the steps for calibrating hand sprayers, and discusses active ingredient recommendations, measuring herbicides and factors that affect spray rate. Calibration of tractor-drawn sprayers is discussed in a separate fact sheet. To assure accuracy, take all measurements carefully and carry all calculations to at least four or five decimal places (i.e. 0.00918 acres). Final measurement of herbicide volume or weight should be done as accurately as equipment will allow.
Step 1: Determine the volume of spray delivered to a known area.
Method 1: Fill the sprayer with a known volume of water. Spray a known area of ground, measured as a fraction of an acre, and measure the amount of water needed to refill the sprayer.
Method 2: Fill the sprayer at least half full with water. Walk a known distance as if spraying but have someone catch the water as it leaves the sprayer nozzle. Measure the volume of water. This is the volume of water used to spray an area equal to the distance walked multiplied by whatever width band you will spray when making the herbicide application.
Step 1 should be performed several times to determine an average volume.
Step 2: Determine the amount of herbicide to add to a particular volume of water to obtain a desired herbicide application rate.
The amount of herbicide to add to a specified volume of water to obtain a desired herbicide application rate can be calculated as follows:
weight of herbicide needed = (64 oz/A) x (0.00918 A)/gal = 0.58752 oz/gal
You would add 0.58752 ounces (dry measure) of herbicide to one gallon of water to spray at a rate of 4 lbs/A.
Weight of Herbicide needed = recommended rate (in pounds or quarts per acre) X acreage covered with the specified volume of water
Calibration for liquid herbicide is similar. There are 128 fluid ounces in a gallon. The calculation for an herbicide recommendation of 1 1/2 qts/A of a liquid herbicide would be as follows:
volume of herbicide needed = (48 oz/A) X (0.00918A)/gal = 0.44064 fl oz/gal
Step 3: Determine the amount of herbicide to add to a full sprayer of water.
Most sprayers used for herbicide applications have capabilities greater than one gallon. Therefore, you must determine the amount of herbicide to add to a full sprayer. This is calculated as follows:
You would need to add 1.4688 ounces of herbicide to the 2 1/2 gallon sprayer to spray at the recommended rate of 4 lbs/A.
Herbicide recommendations are sometimes made in terms of "pounds of active ingredient per acre" (lbs a.i./A) rather than the usual "pounds (or quarts) per acre" of formulated herbicide. When this is the case, the amount of herbicide needed is calculated as follows:
Weight of herbicide to add to full sprayer = weight of herbicide to add to known volume of water X volume of sprayer/known volume of water.
Weight of herbicide to add to full sprayer =0.58742 oz/gal X 2.5 gal/1 gal - 1.4688
You would have needed to add 0.7344 dry ounces to each gallon of water, or 1.836 ounces to the full 2 1/2 gallon sprayer in order to obtain the recommended application rate of 4 lbs a.i./A
When adding herbicides to a sprayer, it is convenient to use either a container which 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).
Amount of herbicide needed = weight of herb/percent active ingredient
Spraying should be done at the same tank pressure as when calibrating. If, for example, a small compression tank is calibrated at 25-30 p.s.i. pressure, spraying should be done at that pressure. Spraying at a higher or lower pressure will result in more or less herbicide being applied as the sprayer passes over the ground.
When using a backpack type sprayer a pressure gauge or pressure regulator will enable you to maintain constant pressure. Of the two, the regulator is more convenient to use. It will automatically control the pressure. When using a gauge, which only indicates the pressure level, pressure must be controlled manually by the rate at which you pump.
Amount of herbicide needed for one gallon = 0.58752 oz/gal / .8 = 0.7344 oz/gal
Amount of herbicide needed for 2.5 gallons = 1.44688 oz/sprayer / .8 = 1.836 oz/sprayer
Generally, even flat spray nozzles are most desirable for band or spot spraying. They provide the most even distribution of spray material across the spray pattern. 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.
Spraying should be done at a uniform speed and at the speed at which the sprayer was calibrated. Slower or faster speeds will result in higher or lower rates of herbicide application. For this reason, hand sprayers, such as the small compression tank sprayers, should be calibrated for use by each individual who uses them. Each person has a different speed and pattern of spraying. One person may, for example, use one gallon to spray 340 square feet while another uses 3/4 of a gallon and another 1 1/2 gallons. Because the terrain affects speed of travel, calibration should be done on the actual area to be sprayed to achieve the most accurate application rate.
Backpack sprayers are commonly used to apply herbicides in forest plantings.
When spraying with a backpack-type sprayer, a pressure regulator (A) or a pressure guage (B) will enable you to maintain constant pressure.
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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