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Extension > Agriculture > Crops > Weed management > Herbicides > How to calculate herbicide rates and calibrate herbicide applicators

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How to calculate herbicide rates and calibrate herbicide applicators

John A. True, Retired Extension Engineer Department of Biosystems and Agricultural Engineering
Reviewed 20081

Uniform application of chemicals at proper rates is essential for effective pest control. A slight variation in the rate of application with some chemicals may result in poor control of the pest or injury to the crop or the environment, causing a loss of time, effort, and money.

Application rates

Herbicide rates may be given in terms of active ingredient or acid equivalent per acre treated, or as pounds or volume of commercial product per acre.

Active ingredient indicates the amount of non-acid herbicide in a formulation. Acid equivalent indicates the amount of an acid herbicide in a formulation.

Herbicides may be applied broadcast (uniformly over the entire field surface) or in bands (narrow strips of herbicide centered over the row with the area between rows left untreated). The rate of application in the treated area should be the same for the band as for the broadcast application. The amount of herbicide needed can be reduced by as much as two-thirds by banding the application over the row and controlling the weeds in the middles with mechanical cultivation.

For example: A 3-pound-per-acre rate requires 3 pounds of herbicide for each crop acre in a broadcast application; but only one pound is needed to apply the herbicide in 10-inch bands on 30-inch rows. To calculate the amount needed for band application, multiply the broadcast rate by the band width divided by the row width.

Herbicide per acre

Commercial products contain differing amounts of active ingredients or acid equivalent. The amount of herbicide formulation to use per acre may be given on the product label. If you have to calculate from the active ingredient amount, use this method:

Divide the pounds per acre of active ingredient by its concentration in the product. If 3 pounds of active ingredient are needed per acre, and the product is an 80 percent powder, then divide 3 by 0.80 to get 3.75 pounds, the amount of powder needed per acre.

For liquids, the concentration may be given in acid equivalent. Then divide the pounds per acre of acid equivalent needed by the pounds per gallon contained in the product.

For example: If 3 pounds-per-acre of acid equivalent are needed, and the product contains 2 pounds-per-gallon, then divide 3 by 2 to get 1.5 gallons-per-acre, the amount of product needed.

How much in the tank?

After you have calibrated the sprayer to know your application rate in gallons-per-acre, divide that number into the gallons applied with each tankful to find the acres covered with each tankful. Then multiply the acres-per-tank by the herbicide needed per acre to find the herbicide to put in each tankful.

For example: Assume you have a 30 ft. sprayer that holds 500 gallons, and you want to apply an herbicide at 3 pounds of active ingredient per acre. The product comes as an 80% wettable powder. You calibrate your sprayer on a 300 foot course and apply 4 gallons of spray to the course.

Your calculations are:

 lb AI/acre   =   3 lb/acre   =   3.75 lb/acre
conc. 0.80
 4 gal/test x 43560   =   19.4 gal/acre
300 ft x 30 ft
 gal/tank   =   500 gal   =   25.8 acre/tank
gal/acre 19.4 gal/acre

3.75 lb/acre  x  25.8 acre/tank  =  96.8 lb/tank

For a granular material that is 50% active ingredient:

 lb AI/acre   =   3 lb/acre   =   6.0 lb/acre
conc. 0.50

The applicator must be set to put on 6 pounds of the granules per acre.

Sprayer calibration

Figure 1. Spray distribution pattern of different nozzle types.

The sprayer must be set to apply the proper amount of total spray mixture. This is usually given in gallons per acre and is listed with the herbicide recommendations or on the product label. Typical amounts range from 10 to 30 gallons per acre.

To check the distribution pattern across the boom, operate over a smooth bare area. Set the boom at the proper height above the ground and spray slowly so the spray wets the ground. If you see streaks as it dries, the boom must be raised or lowered. When the application is even, that boom height is the proper distance to set the boom above your target plants, or ground. If you change nozzles or pressure, the height should be checked again.

To measure the amount applied by the sprayer, follow these steps:

  1. Check the output of all the nozzles to make sure they are flowing at the same rate. If a nozzle is flowing more than 10% over its rated capacity, or more than 5% above or below the average of all the nozzles on the boom, replace it.
  2. With clean water in the tank, set the pressure at the proper level for the nozzles you are using and adjust the nozzle height for uniform coverage.
  3. Spray over a test course in the field at the speed you will use while spraying. Note the throttle setting or speed indicator so you can use the same speed later. Note the time it takes to spray the course, in seconds.
  4. With the sprayer standing still, operate it at the pressure you will be using and collect the liquid from the nozzles over the same time as it took to drive the course and measure the output in gallons.
  5. Figure the gallons per acre:
     gal/acre   =   gal/test x 43560 
    area treateda

Granule applicators

To measure the output of dry material applicators:

  1. Tie plastic bags over the ends of the delivery tubes.
  2. Drive the test course with the applicator running.
  3. Weigh the material collected.
  4. Calculate the rate:
     lb/acre   =   lb/test x 43560b
    area treateda

If you use a flow meter to measure flow through the nozzle, the meter will show gallons-per-minute (GPM) for each nozzle. You calculate gallons-per-acre this way:

 gal/acre   =   GPM/nozzle x seconds/test x 8712c
 course length x nozzle spacing in inches 

for banding:

 gal/acre   =   GPM/nozzle x seconds/test x 8712c
 course length x band width 

aArea treated is the test course length x width; or the course length x band width x no. of bands. b43560 is the number of square feet per acre c8712 is a conversion factor.

Conversion factors

You Have Multiply by: To Get:
cubic feet 7.48 gallons
cups .0625 gallons
liters .2642 gallons
ounces .0078 gallons
pints .125 gallons
# of water .1198 gallons
quarts .25 gallons
You Have Multiply by: To Get:
feet/minute .0114 miles/hour
feet/second .682 miles/hour
You Have Multiply by: To Get:
square feet .000023 acres
acres 43560. square feet
mile 5280. feet
You Have Multiply by: To Get:
lb/acre .367 oz/1000 ft2
lb/acre .023 #/1000 ft2
lb/acre .0023 #/100 ft2
lb/acre .0306 x width oz/1000 ft
lb/1000 ft2 43.56 #/Acre
oz/1000 ft2 2.72 #/Acre
oz/1000 ft 32.67/width #/Acre

1Reviewed by Jonathan Chaplin, Associate Professor Department of Biosystems and Agricultural Engineering

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