Sprayer Calibration: Getting It Right
Pasture is the backbone of Florida’s cattle ranching industry and as such provides the foundation for nutrition of the entire herd. Early in my career as an extension livestock agent, an old Florida Cracker told me “Yes, I’m a cattleman, but I’m a grass farmer first. From improved pasture and hay to silage, I expect my ranch to provide the majority of my cattle’s nutritional needs each year.”
Weed control is a critical component of any pasture management plan. It is important to keep weed populations under control because they compete with desirable forage for space, sunlight, moisture, and plant nutrition. Most ranchers manage their pasture weeds through a combination of cultural, mechanical, and chemical control methods. Herbicides are an important tool in maintaining healthy pastures. And, applying the correct amount of herbicide is critical to achieve optimum weed control and improve forage production. Two of the most common sprayer calibrations are the 5940 method and the 1/128th Acre method.
THE 5940 METHOD: GPA = (5940 X GPM) divided by (MPH X W)
GPM = gallons per minute. To find gallons per minute, catch the discharge from a nozzle for one minute in a container with graduations in milliliters or ounces (1 gallon = 128 ounces = 3785 milliliters). Example: you catch 16 ounces in one minute. 16 ounces divided by minutes times one gallon divided by 128 ounces. The ounces cancel out so, 16 times 1 divided by 128 equals 0.125 gallons per minute.
W = Nozzle spacing. To determine “W”, measure nozzle spacing. Common spacing for boom-type sprayers range between 15 and 24 inches. Boomless sprayers typically have a spray pattern of 12 to 15 feet. When using a boomless sprayer, convert feet to inches.
MPH – Miles per hour. To be precise, it’s advisable to check your speed. Determine the number of seconds required to drive 100 feet. MPH – (distance (ft.) X 60) divided by (time (sec) X 88). Example: it took 16 seconds to travel 100 feet. (100 ft X 60) / 16 sec X 88) = 4.26 MPH. Be sure the engine speed (RPM) and transmission gear are set the same as will be used for applying herbicide. Remember, small changes in ground speed cause large changes in the amount of herbicide being applied. If speed is doubled, gallons per acre decreases by one-half.
5940 = Constant that makes units cancel. Once GPM, W, and MPH have been determined, plug the values into the following formula: (5940 X GMP) / (MPH X W) = GPA. Example: GPM – 0.250 gal/min., W = 20, and MPH = 4.26. (5940 X 0.250) / (4.26 X 20) = 17.43 GPA
1/128th ACRE METHOD:
There are 128 ounces in a gallon; therefore, the number of ounces sprayed per 1/128th acre is equal to the number of gallons sprayed per acre. This method works well for boom-type sprayers, but is less effective for boomless sprayers.
Follow these steps to calibrate a boom-type sprayer using the 1/128 Acre method.
- Determine nozzle spacing and select course length for chart below.
22 24 26 28 30 32 34 36
185 170 157 146 136 127 120 113
- Measure and mark course to be sprayed.
- Record the amount of time required to drive the length of the course. Be sure the engine speed (RPM) and transmission gear are set the same as will be used for applying herbicide.
- Park the sprayer and maintain engine RPM used to drive the measured course. Turn on sprayer.
- Collect all output from one nozzle for the amount of time determined in step 3.
- Measure the ounces caught. Ounces caught is equal to gallons per acre applied.
- To ensure accuracy, repeat steps 5 and 6 for several other nozzles.
BACKPACK / SINGLE NOZZLE CALIBRATION
- Measure and mark a plot 18.5 feet by 18.5 feet
- Have someone time you while you spray the plot with water. Do this three times to get an average time.
- Spray into a container for the average time from step 2.
- Measure the amount collected in container.
- The number of ounces collected is equal to gallons per acre being applied.
For more information about calibration and application of herbicides, visit UF/IFAS Weed Science website at http://weedext.ifas.ufl.edu/