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March 06 LSMP - Close Up - Calculating Herbicide Applications03-02-06 | News



Calculating Herbicide Applications

By Michelle Van Drisse, Writer, Public Relations Group of Bader Rutter & Associates






When calibrating equipment for granular applications, remember that different products may have different granule sizes and densities, so be sure to calibrate each time you apply a different granular product.


Take the guesswork out of calculating herbicide applications before your next treatment with easy-to-read information from Turf & Ornamental Resource Center on the Dow AgroSciences Web site at www.dowagro.com/turf/resource/index.htm. The purpose of Resource Center is to help you achieve a higher success rate with your pesticide program by providing a wealth of practical information and tips, and to be a means by which you can measure your success.

Resource Center begins with a thorough, three-part Conversion Chart for Turf Professionals, which makes it simple to determine proper usage rates by already calculating how much product is needed based on some common dimensions. For quick reference, this handy chart also spells out many commonly used measurements. The Equipment link of Resource Center gives you relevant information on various types of sprayers, nozzles and granular applicators, and is an excellent source for training employees by providing essential factors on how to properly calibrate the equipment.

The Application Guidelines section offers tips on safely making the most of each treatment. And while every product label from the website lists the appropriate Personal Protection Equipment necessary for application, Resource Center is a great reference for a detailed list of ways to protect oneself when applying herbicides. Resource Center also has an alphabetical listing of more than 30 distributors nationwide as a convenient way to find products in your area. With the comprehensive information Turf & Ornamental Resource Center provides, landscape professional can be more knowledgeable about calculating herbicide applications from beginning to end.




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When calibrating equipment for granular applications, remember that different products may have different granule sizes and densities, so be sure to calibrate each time you apply a different granular product.


Lawn Spray Guns

To properly calibrate these sprayers and train your applicators, you need to know these four important factors: Application Rate – The standard rate is measured in the amount of liquid applied per unit area, usually in gallons per 1,000 sq. ft. Several factors can affect this rate: equipment, spray delivery system, product, target, growing conditions and operational considerations.

Your goal is to apply the proper rate consistently in a uniform practice.

  • Swath Width - For lawns, this ranges from three to 10 feet. You can use a series of collection cups or a spray tray to measure the swath width you want. A standardized height, as well as movement with certain handheld spray delivery systems, will assure that a constant swath width is maintained. An effective swath is the total swath width minus the amount of swath overlap.
  • Walking Speed - Once you’ve established the dimensions of your calibration course, here’s how to find out the right walking speed for application. Just time your trainee while covering the course with the effective spray width. Or if you need to find out the speed in miles per hour, use the following formula:
  • Calibration and Flow Rate - Calibration is the process of measuring and adjusting the amount of liquid mixture applied to an area. Use this basic formula: (Calibration course x Flow = gal./1,000 sq. ft. flow rate coverage time.)






Backpack sprayers come in two- or three-gallon compression or continuous pump sprayers. Both are ideal for small jobs or odd-shaped areas where a power unit cannot be used.


Granular Applicators

Granular applicators can be calibrated in turf or other areas off of the application site. You can also use plain fertilizers of the same type that you’ll use in fertilizer combination products. For manually operated spreaders, the person who does the actual application should be the one who calibrates the equipment.

Follow these simple steps:

  1. Mark out a test strip of 440 square feet (approximately 1/100th of an acre).
  2. Weigh your granular product before and after you apply.
  3. Subtract the difference to get the amount applied, then multiply by 100 to get the amount you need per acre.
  4. Adjust your equipment and repeat the test until you have the correct setting for the required amount per acre.

Different products may have granules of different sizes and densities. Also, they may change the size of its granules from one year to the next. So you should repeat your calibration procedure each time you apply a different granular product. Contact a spreader manufacturer for details.






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Backpack Sprayers

All sprayers require calibration. Just add the amount of control product you need to treat the area with enough water to spray uniformly over the area at least twice. (You can add a marker dye to help assure uniform application. The dye breaks down in sunlight and disappears in three or four days.)

Here’s how to calibrate your sprayer to treat landscaped areas. Follow these five steps:

  1. Find an area where you can easily mark a test plot. Create a test plot with a well-defined area. A good rule of thumb is to use 1,000 square feet. Test plots may be 10 feet by 100 feet; 5 feet by 200 feet; 20 feet by 50 feet; or any combination to reach your predetermined area.
  2. Completely fill the sprayer with water only. (You’ll need to know the total capacity of the sprayer.) Add marker dye.
  3. While pumping to maintain the selected application pressure, walk at a steady pace and spray the dyed water evenly over the test area. It’s a good idea to spray the area twice to assure uniform coverage.
  4. After you have sprayed the test area twice, again fill the sprayer up with water, measuring how much it takes. This is the volume of liquid needed to cover the test area. If you sprayed 1,000 square feet and it took two gallons to refill the sprayer, then you can use 2 gallons of spray mixture for every 1,000 square feet.
  5. You can then refer to product labels to determine rates for mixing.

The Surflan rate you plan to use is 3 ounces per 1,000 square feet. If your sprayer used 2 gallons to spray this area, you would mix 3 ounces of Surflan for every 2 gallons. A 4-gallon tank would need 6 ounces of Surflan to cover 2,000 square feet.

Boom Type/Tractor Mount Sprayers






To spray uniformly, adjust flat fan nozzles so that 30 to 50 percent of the spray pattern overlaps. The middle sprayer covers a total of 30 feet across, with 10 feet of overlap to the right and left sprayers.


For larger areas such as field nurseries, golf fairways, or other large turfgrass areas, follow these seven steps to calibrate your sprayer:

  1. Find an area that best represents the average topography for the area to be sprayed. Measure it and mark off the calibration distance that coincides with your nozzle spacing or your band width. Calibration distance = __ feet.
  2. Walk or drive across the area, maintaining an even speed. If using a tractor-mounted sprayer, notice what the engine RPM is, plus what gear, and write it down so that you use the same speed during calibration and application. RPMs gear.
  3. Record the number of seconds it takes to travel the calibration distance and write it down. __ seconds.
  4. Fill your sprayer, engage the pump and adjust the pressure regulator to the boom pressure you want (between 15 and 50 psi for pesticides). Collect all the water from one nozzle for the same number of seconds you have written above. With a 20-inch nozzle spacing, if it took 35 seconds to travel 204 feet, collect the discharge of one nozzle for 35 seconds. The number of fluid ounces collected equals the gallons per acre (GPA) of output of that nozzle. (20 ounces collected equals 20 GPA).
  5. Repeat Step 4 two more times, collecting water from a different nozzle each time. The average number of ounces collected from each of the three nozzles is equal to the gallons of water applied per acre for that boom. Also remember to maintain the same pressure and travel speed when spraying.
  6. Divide the capacity of the tank by the number of gallons of water applied per acre (GPA) to find out the area (in acres) you can cover with a tankful of spray. 200 gal./tank divided by 20 GPA = 10 acres covered/tank.
  7. To find out how much control product you need to add to the spray tank, multiply the application rate of the product per acre by the acres covered per tank. Then add that amount of pesticide to your sprayer tank. Two quarts per acre x 10 acres/tank = 20 quarts., or 5 gal./tank.






Tractor-mounted sprayers, as seen in this photo, are designed to spray larger areas with higher spray volumes at either low or high pressure. Measure it and mark off the calibration distance that coincides with your nozzle spacing or your band width.



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