Brad Withrow-Robinson, Forestry & Natural Resources Extension, Benton, Linn and Polk Counties. Photos by Jody Einerson, Benton County Extension.

In many of our herbicide application situations, the objective is to treat an area, such as a plantation of young trees, at a specific application rate.  By rate, we mean applying a specific amount of a material over a given area (e.g. ounces of product per treated acre).  This is essential when applying soil active herbicides, and also for area treatments (as opposed to targeted spot treatments) with foliar herbicides, particularly if the spray mix will be contacting the seedlings, such as in “over the top” applications. Applying too little material will not give you effective weed control, wasting time and money.  Too much material risks killing seedlings and other desirable plants, or causing other environmental damage.  Figuring out how much spray you are applying so that you can calculate how much herbicide to mix into the spray tank is called calibration.

Once you have calibrated your backpack sprayer, you can apply spray in any number of patterns, and still be putting out an accurate amount of material on a per acre basis.  That means you can accurately treat the entire plantation (broadcast), or discreet parts of that plantation such as row strips or patches around trees with confidence. 

Calibration requires a few simple activities and calculations, in two steps.  Here is a simplified refresher:

Step 1: Find the Application Rate (Gallons/Acre, gpa) you apply

  • Measure a 100 foot  (or longer) track
  • Put a precise amount of water in the sprayer (typically 1-3 gallons)

Volume of spray (water) =  _____  gal

  • While walking at a comfortable, sustainable pace, and while pumping to maintain a steady pressure, go back and forth on track until sprayer is empty.  Figure and record the distanced travelled (sprayed).

Spray distance (travelled) = _________ feet

  • Measure width of your band (for a given nozzle type, held at a specific height) in feet

Spray (band) width  = ­­­­­_________ feet

  • Calculate area sprayed (part of an acre) by multiplying spray distance by band width, then dividing by sq feet in an acre.

spray dist. (ft) * spray width (ft)  =  ________*________  = _______   acre                     43,560 ft2/acre                               43,560 ft2/acre                

  • Calculate the application rate (gallons per acre; gpa) by dividing your calibration volume sprayed by your area sprayed.

Application rate  =  Volume sprayed (gal)   =  __________  =  __________  gpa . Area sprayed (acres)

You typically want to be applying between 10 and 20 gallons per acre.  This helps with getting good spray coverage.  It is also a label requirement for some products to be applied in at least 10 gallons of spay mix per acre.  If not within the target, you need to change something (usually a different size spray nozzle, or walking pace if close) and measure again, to fall within the appropriate range.

Step 2: Determine the herbicide mixture(s)

Having determined your application rate, you now need to calculate the amount of herbicide (and adjuvant) to add per gallon.

  • On the label, find the application rates for the product(s), in amount of the product per acre. 

For liquids this rate could  be ounces, pints, quarts or even gallons per acre.

  • Convert to ounces of product per acre by multiplying the number of units (say pints) by the number of ounces in that unit (would be 32 for a quart of liquid): 

(No. units/acre) * (oz in unit) =  ________ * ______   =   ________  oz/acre

For dry formulations, rates will commonly be given in ounces or pounds of product per acre. If in pounds, convert to ounces product per acre: 

(No. pounds prod./acre) * (16 oz/lb) = ________ *  16 = ________  oz/acre

  • Calculate amount of product to add per gallon of spray by dividing amount of product (number of ounces per acre) by the spray application rate:

 Product   (oz/acre)__    =          ______     =     _______ oz/gal    . Application rate (gpa)

For liquids, a measuring cup gives you the liquid measure. But for dry products a regular measuring cup won’t do, since it measures volume not weight. Some dry products come with a specially calibrated measuring cup to deliver a weight of the product with a dry volume measure.  A small (postal) scale is a better tool for small amounts, as used in a backpack sprayer.

That is my Calibration Refresher. Note that these numbers are particular to you, for you walking and pumping pace, for that particular nozzel tip and height. They do not apply to your sister. She needs to do her own calibration!

Remember, if using herbicides to read and follow the label, wear appropriate personal protection equipment (also in the label), and submit your Notification to ODF ahead of the spray season.  We have previously posted Resources in the Fight Against Weeds and other stories about weed control .

You can find more calibration information, and other approaches online. You might start with this 15 minute video:  Calibrating and Using Backpack Sprayers .  It is less important which approach you use than that you find one that works for you, and that you do a calibration at the start of your project.

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