Backpack Sprayer Calibration for Woodland Applications

Calibration simply means determining the output of a sprayer so you can calculate your "application rate"—the amount of total spray mixture that is applied to a given area, usually measured in gallons per acre. Often, herbicide applications do not involve spraying an entire, contiguous acre. Instead, applicators should think about application rates in terms of "treated" acres, the collective area that was actually sprayed. The amount of treated area is calculated based on two factors: the application rate and the volume of spray that was applied. For example, if you spray at a rate of 20 gallons per acre and you used 10 gallons of spray mixture, then you treated a total of ½ acre (10 gallons of spray ÷ 20 gallons per acre = 0.5 acre).

The "herbicide rate" (also known as "product rate" or "dosage") found on the product label is the amount of herbicide product that is applied to a given area and is often written as fluid ounces, pints, or quarts per acre, for liquid herbicides, and ounces or pounds per acre for dry herbicides. This is different from the "application rate" (the total amount of spray mixture applied to an area) because the application rate includes all of the water, herbicide(s), and adjuvants. Each applicator applies herbicide in their own unique manner when spraying. In order to apply herbicide at a specific dosage, the amount of herbicide that one applicator should include in their spray mixture is calculated based on that individual's specific application rate. Failure to calibrate spray equipment can result in misapplication of herbicides, repeat applications, damaged nontarget plants, excess costs, and environmental concerns.

To apply herbicides in a consistent manner, an applicator needs to become accustomed to seeing a certain amount of spray droplets on the foliage of target plants, known as "spray coverage." Applicators may have different opinions of what adequate coverage looks like; therefore, it is important to have each applicator calibrate individually, even if they use the same equipment as another applicator. If Applicator A applies twice as much spray mixture to a given area as Applicator B, then A must put half as much herbicide into their mix in order to apply the same herbicide dosage as Applicator B.

Ounces-to-Gallons

When calibrating a sprayer, the process of determining an application rate can be simplified by using the "Ounces-to-Gallons" method. With this method, the amount of spray, measured in fluid ounces, applied to the calibration area converts directly to gallons per acre. It is based on making the calibration application to a scaled-down area of 1/128th of an acre, or 340 square feet (43,560 square feet per acre ÷ 128 = 340 square feet).

We do this because 1 gallon equals 128 fluid ounces. By treating 1/128th of an acre (340 square feet), we can directly convert a measurement of ounces of water sprayed on the calibration area to gallons sprayed per acre. For example, applying 30 ounces to 340 square feet is the same coverage as applying 30 gallons to an acre. Once we have determined our application rate in gallons per acre using this method, we can determine how much herbicide to include in the spray mixture. This method can be used to calibrate four types of backpack sprayer applications common in woodland settings:

• Broadcast Application (Spraying a large contiguous area of relatively even vegetation)
• Spot Application (Spraying target plants that are scattered and variable in size and shape)
• Single-seedling Plot Application (Spraying a small patch of vegetation where a tree will be planted or around a tree shelter)
• Band Application (Spraying a continuous, fixed-width strip on the ground or on low vegetation)

This fact sheet will describe the calibration process for these four types of applications in the following section. Additional sections explaining herbicide calculations for mixing and general mixing guidelines can be found below the calibration section.

Calibration

Broadcast Application

What you need:

• Backpack sprayer equipped with spray wand or gun and adjustable cone nozzle
• 50-foot measuring tape or measuring wheel
• Measuring cup marked to 30–40 fluid ounces
• Pencil or marker
• Objects to mark calibration area (flags, cones, etc.—at least 4)
• Water (mixed with colored dye if calibrating on vegetation)

With a broadcast application, the applicator treats a large, contiguous area of low- to medium-height vegetation. In forestry, broadcast applications are commonly used for spraying areas of interfering plants, such as hay-scented and New York fern, as well as infestations of some invasive plants, including Japanese stiltgrass and mile-a-minute vine.

Additionally, these applications can be used for site preparation prior to a tree- or meadow-planting, or to apply preemergence herbicide to an area.

This section covers the calibration process for a broadcast application using a backpack sprayer and an adjustable cone nozzle, which requires sweeping the nozzle from side to side as you spray. Note that a large area can also be treated using nozzles that produce a fixed-width pattern, which would not be moved from side to side while spraying. If you are using a fixed-width setup, see the section for "Band" applications.

Step 1: Practice proper technique
Fill your backpack with water, then pump up the sprayer and adjust the spray nozzle to a mid-range setting that produces a somewhat solid cone.

Choose an area on an even surface where droplets will be visible, such as pavement or gravel. You could also use colored water and practice on low-growing vegetation.

Practice walking forward and spraying swaths in front of you by moving your spray wand or gun from side to side at a comfortable width. You must maintain consistent spray coverage by pumping the lever steadily, moving steadily, and keeping the spray wand or gun at a consistent height. Direct the spray outward approximately 6 to 8 feet in front of you.

Be sure to release the trigger at the end of each sweep of your arm, or the edges will receive heavier coverage. Check the droplet pattern on the ground to ensure it is being evenly covered and there are no skips. The goal is to apply even but light coverage (where individual droplets can be seen) rather than solidly painting the ground. Applying lightly allows you to cover a larger area with each tank of spray.

Step 2: Lay out the calibration area
Once you feel comfortable maintaining a consistent pressure, swath width, and coverage, it's time to lay out the calibration area. Start by measuring your swath width (spray one swath several times to make it more visible), then divide 340 square feet by your width to calculate the needed length of the calibration area. For example, if your swath width is 10 feet, the length of the calibration area should be 34 feet (340 ÷ 10 = 34).

Step 3: Measure your water
After laying out the calibration area and ensuring the spray nozzle is at the desired setting, make sure the sprayer is fully pressurized, then check that there is at least 1 gallon of water in the spray tank. Mark the level of water in the tank. You can fill it to a certain line already on the tank or draw a line with a pencil or marker (ink can be removed with acetone on a plastic sprayer reservoir). Note that the sprayer must be pressurized, so the pump and hose are already filled with water before you mark the water level. Otherwise, pressurizing the sprayer will lower the starting level below your mark.

Also, fill a measuring cup to a known level with water (30 to 40 fluid ounces should be sufficient).

Step 4: Spray the calibration area and re-measure the water

Spray the calibration area in the same manner as your practice runs. Then, using the measuring cup, refill your sprayer to the original starting level and use subtraction to determine how many fluid ounces you applied. For example, if the cup started with 32 ounces and had 7 ounces left after you refilled the sprayer to the starting point, then you sprayed 25 ounces (32 – 7 = 25). Because you used a calibration area of 340 square feet, this means your application rate was 25 gallons per acre.

Step 5: Repeat for an average rate
It is unlikely that you will get the same exact rate every time, so repeat the calibration process at least 3 times to get a reliable average. Each calibration should produce a rate within a range of about 10 percent of the other calibrations. If you have a wider range of application rates, keep practicing and recalibrating until you get the same rate, plus or minus 10 percent.

For example, say the first calibration produces a rate of 20 gallons/acre. Ten percent of 20 is 2 gallons (0.1 x 20 = 2). So, the second and third calibrations should be no less than 18 gallons and no more than 22 gallons per acre. Once you have at least 3 calibrations within a 10 percent range, calculate the average of all the rates and use that average rate for your mixing calculations.

Spot Application

What you need:

• Backpack sprayer equipped with spray wand or gun and adjustable cone nozzle
• 50-foot measuring tape
• Measuring cup marked to 30–40 fluid ounces
• Pencil or marker
• Flagging tape (if calibrating on "wall" of vegetation, like shrubs)
• Objects to mark calibration area, if calibrating on level area of low-growing vegetation (flags, cones, etc. —at least 4)
• Water mixed with colored dye

A common use for a backpack sprayer in woodlands and natural areas is to "spot treat" scattered individual plants, or patches of plants, from small herbaceous weeds to shrubs and small trees. Similar to a broadcast application, spot treatments rely on the applicator becoming visually attuned to the spray coverage on the leaves. Calibrating for spot treatments allows the applicator to use visual information to avoid over- or under-applying. Remember to include colored dye in your mixture so you can see the droplets on the plants.

Step 1: Practice proper technique
First, choose an area to practice spraying with water and colored dye, ideally on a variety of vegetation, from larger shrubs to small herbaceous plants. Pump up the sprayer and adjust the spray nozzle to a mid-range setting that produces a somewhat solid cone. You should be able to produce optimal spray coverage by holding the nozzle approximately 6 feet from the target plants. Spray until you become consistent at applying a certain visual coverage, aiming for even and light coverage where the individual droplets are visible on the leaves. Spraying lightly allows you to treat more vegetation with each tank of spray mixture. For spot applications, it is best to spray in short bursts rather than holding the trigger open.

Step 2: Lay out the calibration area
Recall from the Ounces-to-Gallons section at the beginning of this fact sheet that a calibration area of 340 square feet will simplify the math. Ideally, the calibration area should be laid out on vegetation similar to what will be sprayed in the field. This could be an area of low vegetation on the ground or a "wall" of vegetation, such as shrubs along a forest edge. For the wall option, choose a height that can be comfortably reached for spraying, then calculate the length of the area. For example, if you choose a height of 7 feet, the length should be 48.5 feet (340 ÷ 7 = 48.5). Mark the calibration area using flagging tape attached at the chosen height and spaced along the length of the area.

For low vegetation, choose a comfortable width and calculate the length to make 340 square feet, then mark the corners of the calibration area with flags, cones, or other objects.

Step 3: Measure your water
After laying out the calibration area and ensuring the spray nozzle is at the desired setting, make sure the sprayer is fully pressurized, then check that there is at least 1 gallon of water in the spray tank. Mark the level of water in the tank. You can fill it to a certain line already on the tank or draw a line with a pencil or marker (ink can be removed with acetone on a plastic sprayer reservoir). Note that the sprayer must be pressurized, so the pump and hose are already filled with water before you mark the water level. Otherwise, pressurizing the sprayer will lower the starting level below your mark.

Also, fill a measuring cup to a known level with water (30 to 40 fluid ounces should be sufficient).

Step 4: Spray the calibration area and re-measure the water
Spray the entire calibration area with the colored water in the same manner as you practiced, focusing on maintaining consistent spray coverage that is repeatable on individual plants in the field. Then, using the measuring cup, refill your sprayer to the original starting level and use subtraction to determine how many fluid ounces you applied. For example, if the cup started with 32 ounces and had 7 ounces left after you refilled the sprayer to the starting point, then you sprayed 25 ounces (32 – 7 = 25). Because you used a calibration area of 340 square feet, this means your application rate was 25 gallons per acre.

Step 5: Repeat for an average rate. It is unlikely that you will get the same exact rate every time, so repeat the calibration process at least 3 times to get a reliable average. Each calibration should produce a rate within a range of about 10 percent of the other calibrations. If you have a wider range of application rates, keep practicing and recalibrating until you get the same rate, plus or minus 10 percent. For example, say the first calibration produces a rate of 20 gallons/acre. Ten percent of 20 is 2 gallons (0.1 x 20 = 2). So, the second and third calibrations should be no less than 18 gallons and no more than 22 gallons per acre. Once you have at least 3 calibrations within a 10 percent range, calculate the average of all the rates and use that average rate for your mixing calculations.

For spot applications, it may be helpful to do at least one calibration on each type of vegetation (low-growing and "wall").

Single-seedling Plot Application

What you need:

• Backpack sprayer equipped with spray wand and "conventional flat-fan" nozzle
• Measuring tape (at least 6 feet)
• Measuring cup marked to 30–40 fluid ounces
• Pencil or marker
• Object to mark plot center (cone, roll of tape, rock, etc.)
• Water

When planting trees, it is important to control weed and grass competition prior to planting, and then annually for several years after planting. Treating unwanted vegetation with herbicide in a spot where the tree is, or will be, planted greatly improves seedling survival and growth by reducing plant competition. It also reduces cover for rodents, particularly meadow voles, that may gnaw on young trees. Most hardwood tree plantings protect tree seedlings with 4- to 5-foot-tall plastic tube shelters, which are placed over the seedlings and staked into the ground. Tree shelters protect the seedlings from browsing animals and facilitate spraying by protecting the seedling from the herbicide mixture. Note that you can choose the size of plot to spray around each tree, but a standard practice is to spray a 6x6-foot plot centered on the seedling, so those are the dimensions that will be used in this example.

Step 1: Practice proper technique
Make a single-seedling plot application using a backpack sprayer equipped with a spray wand and a "conventional flat-fan" nozzle, which applies less liquid at the edges of the spray pattern. This is different from an 'even' flat fan nozzle, which does not taper at the edges. A nozzle with a spray angle of 110 degrees is recommended.

Using a paved or gravel surface, place an object, such as a roll of flagging or tape, on the ground to mark the center of the plot.

To treat a 6x6-foot plot, apply two parallel spray bands, one on each side of the tree spot, both 6 feet long and 3 feet wide. Each pass of the spray wand should be centered 1.5 feet from the shelter. Hold the nozzle approximately 2 feet above the ground so the pattern extends about 6 to 9 inches past the shelter into the footprint of the other band. This overlap of the two spray bands is necessary for uniform coverage across the plot when using a conventional flat-fan nozzle.

Using plain water or water mixed with colored dye, practice spraying the targeted area on both sides of the tree spot. Focus on covering the full 6-foot length, maintaining consistent speed and nozzle height above the ground during each pass. When the spot is dark enough for easy visibility, measure the dimensions to ensure accuracy.

Keep in mind that the practice spraying and calibration will produce much heavier spray coverage than an actual application would because you are spraying one plot multiple times. It may be helpful to use some colored water to practice spraying vegetation around a few tree shelters (or other objects) as you actually would in a real application, that is, spraying each "band" one time. This can help you see how light the spray coverage will actually be.

Step 2: Calculate the calibration plot
The calibration is based on the Ounces-to-Gallons method described at the beginning of this fact sheet, using a 340-square-foot calibration area. However, you won't be measuring out an entire area of 340 square feet because you are limited to the dimensions of the seedling plot. Instead, you must calculate how many times to spray your small plot to reach a total of 340 square feet. If you are treating a 6x6-foot plot, which equals 36 square feet (6 x 6 = 36), you must determine how many 36-square-foot plots make up a 340-square-foot area. Divide 340 by 36 (340 ÷ 36 = 9.4, round to 9.5). This means you will spray the whole 6x6-foot plot 9.5 times to equal 340 square feet. Keep in mind, a 6x6-foot "plot" is comprised of two "bands," so 9.5 plots mean you have to spray 19 individual bands (9.5 x 2 = 19).

Remember, this is based on the suggested plot size of 6x6 feet. If you choose to spray a different-sized plot around your seedlings, you will need to do this math for your plot size. Simply divide 340 by the square footage of your plot, then multiply that number by 2 to calculate the number of individual bands you must spray to equal 340 square feet in total.

Step 3: Measure your water
Make sure the sprayer is fully pressurized and there is at least 1 gallon of water in the spray tank. Mark the level of water in the tank. You can fill it to a certain line already on the tank or draw a line with a pencil or marker (ink can be removed with acetone on a plastic sprayer reservoir). Note that the sprayer must be pressurized, so the pump and hose are already filled with water before you mark the water level. Otherwise, pressurizing the sprayer will lower the starting level below your mark.

Also, fill a measuring cup to a known level with water (30 to 40 fluid ounces should be sufficient).

Step 4: Spray the calibration area and re-measure the water
Spray the calibration plot the number of times that you calculated in Step 2. Then, using the measuring cup, refill your sprayer to the original starting level and use subtraction to determine how many fluid ounces you applied. For example, if the cup started with 30 ounces and had 16 ounces left after you refilled the sprayer to the starting point, then you sprayed 14 ounces (30 – 16 = 14). Because you used a calibration area of 340 square feet, this means your application rate was 14 gallons per acre.

The amount of water sprayed onto this calibration spot will probably be small, and the level change in the backpack tank may be difficult to measure. To make measuring easier, you could double or even triple the calibration area and then divide the ounces used by 2 or 3, respectively, to get the ounces sprayed for 340 square feet. For example, you could double the area by spraying the 6x6-foot plot 19 times (applying 38 individual bands) and then divide the ounces sprayed by 2. So, if your calibration application uses 28 ounces, that means you applied 14 ounces to 340 square feet (28 ÷ 2 = 14) and, therefore, sprayed at 14 gallons per acre.

Step 5: Repeat for an average rate
It is unlikely that you will get the same exact rate every time, so repeat the calibration process at least 3 times to get a reliable average. Each calibration should produce a rate within a range of about 10 percent of the other calibrations. If you have a wider range of application rates, keep practicing and recalibrating until you get the same rate, plus or minus 10 percent.

For example, say the first calibration produces a rate of 14 gallons/acre. Ten percent of 14 is 1.4 gallons (0.1 x 14 = 1.4). So, the second and third calibrations should be no less than 12.6 gallons and no more than 15.4 gallons per acre. Once you have at least 3 calibrations within a 10 percent range, calculate the average of all the rates and use that average rate for your mixing calculations.

Band Application

What you need:

• Backpack sprayer equipped with spray wand and "even flat-fan" or "off-center flat-fan" nozzle (or handheld spray boom with two or more "conventional flat-fan" nozzles)
• 100-foot measuring tape or measuring wheel
• Measuring cup marked to 30–40 fluid ounces
• Objects to mark start and end of calibration strip (flags, flagging tape, cones, etc.)
• Water (mixed with colored dye if calibrating on low vegetation)
• If using the Subtraction Method:
   - Pencil or marker
• If using the Timing Method:
   - Stopwatch
   - A helper to hold the stopwatch

Backpack sprayer band applications are commonly made using a wand equipped with an "even flat-fan" or "off-center (OC) flat-fan" nozzle, though it can also be done using a handheld spray boom with two or more "conventional flat-fan" nozzles. Band applications spray a continuous, fixed-width strip of ground or low-growing vegetation in front of or alongside the applicator while the applicator walks, maintaining steady pressure. In forestry, these applications are mainly used to treat along fence lines, trails, and forestry roads, but are adaptable to a range of use sites. Additionally, band applications can be used to prepare rows for tree plantings or to treat large areas as an alternative to broadcast applications.

Calibrating for a band application uses the Ounces-to-Gallons framework explained at the beginning of this fact sheet, using a 340-square-foot calibration area

Step 1: Practice proper technique
Begin by practicing your technique to apply a consistent and uniform spray pattern with plain water (or water mixed with colored dye if practicing on vegetation). The main variables affecting application rate with band treatments are speed, pressure, nozzle type, and height above the ground or vegetation. Practice walking at a comfortable speed and steadily pumping the lever while using the same nozzle that you will use for the actual spray application, keeping it at a consistent height above the ground. To maintain steady pressure, make sure you don't walk faster than you can pump. If you do future band applications using a different type of nozzle, you will need to recalibrate with that nozzle.

Step 2: Lay out the calibration area
Once you have become comfortable with the application, pressurize the sprayer and hold the nozzle at the same height above the ground as in your practice runs. Without moving the nozzle, squeeze the trigger for several seconds. Measure the width of the resulting spray band, then calculate the required length of your calibration area by dividing the width into 340 square feet. If your band width is 3 feet, the length of the calibration area will be 113 feet (340 ÷ 3 = 113.33, round to 113). Again, doing the calibration application on 340 square feet allows you to directly convert the number of ounces applied to the calibration area to a gallons-per-acre application rate.

Now that you've practiced your technique (Step 1) and have measured out the calibration area (Step 2), it's time to do the actual calibration. For band applications, the calibration can be done in one of two ways, using the Subtraction Method or the Timing Method, each of which is described below.

Subtraction Method

Step 3: Measure your water
Make sure the sprayer is fully pressurized and there is at least 1 gallon of water in the spray tank. Mark the level of water in the tank. You can fill it to a certain line already on the tank or draw a line with a pencil or marker (ink can be removed with acetone on a plastic sprayer reservoir). Note that the sprayer must be pressurized, so the pump and hose are already filled with water before you mark the water level. Otherwise, pressurizing the sprayer will lower the starting level below your mark.

Also, fill a measuring cup to a known level with water (30 to 40 fluid ounces should be sufficient).

Step 4: Spray the calibration area and re-measure the water
Spray the entire calibration area in the same manner as you practiced, focusing on maintaining consistent walking speed, spray pressure, and nozzle height. Then, using the measuring cup, refill your sprayer to the original starting level and use subtraction to determine how many fluid ounces you applied. For example, if the cup started with 32 ounces and had 7 ounces left after you refilled the sprayer to the starting point, then you sprayed 25 ounces (32 – 7 = 25). Because you used a calibration area of 340 square feet, this means your application rate was 25 gallons per acre.

Step 5: Repeat for an average rate
It is unlikely that you will get the same exact rate every time, so repeat the calibration process at least 3 times to get a reliable average. Each calibration should produce a rate within a range of about 10 percent of the other calibrations. If you have a wider range of application rates, keep practicing and recalibrating until you get the same rate, plus or minus 10 percent.

For example, say the first calibration produces a rate of 20 gallons/acre. Ten percent of 20 is 2 gallons (0.1 x 20 = 2). So, the second and third calibrations should be no less than 18 gallons and no more than 22 gallons per acre. Once you have at least 3 calibrations within a 10 percent range, calculate the average of all the rates and use that average rate for your mixing calculations.

Timing Method

Step 3: Time the calibration application
To use the Timing Method, pressurize the sprayer while standing about 10 feet behind the starting point of your calibration area. Note: You should have a helper to hold the stopwatch for this method. When ready, start walking and spraying at your practiced pace. Have your helper start the stopwatch as your spray nozzle crosses the starting point of the calibration area. Spray the entire measured length of the calibration area and stop the timer as the nozzle crosses the endpoint.

Step 4: Measure the water used
Now, hold the spray nozzle over an empty measuring container and spray into the container for the same amount of time that it took to cover the calibration area. Be sure to pump the lever steadily to maintain the same pressure as when you made the application. This is a direct way to measure the number of ounces sprayed onto the calibration area. For example, let's say you ended up with 19 fluid ounces in the measuring cup. Because you used a calibration area of 340 square feet, this means your application rate was 19 gallons per acre.

The Timing Method can only be used for band applications because the spraying is continuous (the spray is never shut off during the application). This method of calibration does not work for the other application types described in this publication because the spray is shut off periodically during those applications.

Step 5: Repeat for an average rate
It is unlikely that you will get the same exact rate every time, so repeat the calibration process at least 3 times to get a reliable average. Each calibration should produce a rate within a range of about 10 percent of the other calibrations. If you have a wider range of application rates, keep practicing and recalibrating until you get the same rate, plus or minus 10 percent.

For example, say the first calibration produces a rate of 20 gallons/acre. Ten percent of 20 is 2 gallons (0.1 x 20 = 2). So, the second and third calibrations should be no less than 18 gallons and no more than 22 gallons per acre. Once you have at least 3 calibrations within a 10 percent range, calculate the average of all the rates and use that average rate for your mixing calculations.

Determining the Herbicide Mixture

Now that you have calibrated and determined your gallons per-acre application rate, the next step is to calculate how much herbicide to include in your spray mixture. Most herbicide rates on product labels are written as an amount of product per acre, usually ounces, pints, or quarts for liquid formulations. Knowing your calibrated application rate in gallons per acre allows you to calculate the proper amount of herbicide to mix. For example, if a product label calls for applying 10 ounces of herbicide per acre, and, based on your calibration exercise, your application rate is 15 gallons per acre, then you would simply mix 10 ounces of the herbicide in 15 gallons of total spray mixture.

If treating a large area, you could make a large batch of herbicide mix in a bigger tank and use it to refill your sprayer. However, you may want to scale down the amount of spray mixture to the capacity of a backpack sprayer, often 3 or 4 gallons, especially if you are treating a smaller area. Regardless of how much spray mixture you decide to make, you can use your application rate, the labeled herbicide rate, and the total number of gallons of spray mixture that you are making to calculate how much herbicide product to include in your mixture.

There are two ways to calculate this: the Amount-per-Gallon Method and the Acres Method.

Herbicide Calculation: Amount-per-Gallon Method

To use the Amount-per-Gallon Method, start by dividing the labeled or prescribed herbicide rate, in ounces per acre, by your calibrated application rate in gallons per acre. This will give you the number of ounces of herbicide to mix per gallon of spray mixture. If the label lists the herbicide rate in pints or quarts per acre, convert this to ounces by multiplying the number of pints by 16 (the number of ounces in one pint) or the number of quarts by 32 (the number of ounces in one quart).

Example:
For this example, we will use a typical fern and stiltgrass broadcast treatment requiring 2 ounces per acre of a sulfometuron herbicide (e.g., Oust XP). To calculate ounces of herbicide per gallon of spray mixture, divide the herbicide rate per acre, in ounces, by the number of gallons applied per acre as determined from the calibration exercise for a broadcast application. Note: For granular herbicides, such as Oust XP, ounce measurements refer to weight rather than liquid volume.

From our Broadcast Application calibration exercise, suppose our application rate is 25 gallons per acre. Calculate ounces of herbicide per gallon of spray mixture:
  2 ounces per acre (labeled herbicide rate)
  ÷ 25 gallons per acre (calibrated application rate)
  = 0.08 ounces of sulfometuron herbicide per gallon of total spray mixture.

Now, simply multiply the amount per gallon by the total number of gallons you will mix. To mix 3 gallons in a backpack sprayer, multiply the above number by 3:
  0.08 ounces × 3 gallons
  = 0.24 ounces of sulfometuron herbicide mixed within 3 total gallons of spray mixture.

Now you are ready to measure out the amount of herbicide needed and mix it in your sprayer.

Herbicide Calculation: Acres Method

Another way to determine how much herbicide to include in your spray mixture is to calculate how many acres (or fractions of an acre) you can treat with the amount of mix you make. Multiplying the herbicide rate (in ounces per acre) by that number of acres (or fraction of an acre) will tell you the amount of herbicide to include in the mix. This method can save a bit of time by requiring one less step than the amount-per-gallon calculation and can be especially helpful if using more than one herbicide in your mix.

Example:
For this example, we’ll use a combination of glyphosate (3 quarts/acre), triclopyr (2 quarts/acre), and metsulfuron (1 oz./acre), which will allow us to control almost any target plant that we encounter. From our Spot Application calibration exercise, we determined our application rate is 20 gallons per acre. We will make a total of 12 gallons of mix in a large tank, which will be used to fill several backpack sprayers (Note: Each applicator has calibrated and practiced so they are all applying at the 20-gallon/acre rate.) Since we're applying at 20 gallons per acre, that means 20 gallons of spray would treat one acre. Since we are only mixing 12 gallons, we'll be treating less than one acre. To calculate the acres (or acre fraction in this case) that we are mixing for, divide the number of gallons we are making by our gallons-per-acre application rate:

   Acres (or acre-fraction): 12 gallons of mix
   ÷ 20 gallons/acre = 0.6 acres

Therefore, when applied at the 20-gallon/acre rate, 12 gallons of mix will treat 0.6 of an acre. This is our acre-fraction. Now that we have our acre-fraction, we can determine the amount of each herbicide to include in the mix. First, to make measuring easier, we will convert the herbicide rates from quarts/acre to fluid ounces/acre:

   Glyphosate: 3 quarts/acre x 32 fl. oz./quart = 96 fl. oz./acre
   Triclopyr: 2 quarts/acre x 32 fl. oz./quart = 64 fl. oz./acre
   Metsulfuron: 1 oz/acre (no conversion needed)

If we were making 20 gallons (the amount needed to treat one acre) then we would include 96 ounces of the glyphosate, 64 ounces of triclopyr, and 1 ounce of metsulfuron in the mix. Since we are making less than 20 gallons, we simply multiply our acre-fraction by each herbicide rate to calculate the amount of each herbicide needed:

   Glyphosate: 0.6 acres x 96 oz./acre = 57.6 ounces round to 58 ounces
   Triclopyr: 0.6 acres x 64 oz./acre = 38.4 ounces round to 38 ounces
   Metsulfuron: 0.6 acres x 1 oz./acre = 0.6 ounces
    (Note: Metsulfuron is a granular herbicide, so this is 0.6 ounces by weight.)

In our 12 gallons of mix, we will include 58 ounces of glyphosate, 38 ounces of triclopyr, and 0.6 ounces of metsulfuron. This method is a bit simpler because each herbicide rate can be multiplied by one number, the acres, or acre-fraction. Both calculation methods, Amount-per-Gallon and Acres, ultimately provide the same results. Whichever you choose is simply a matter of preference.

Guidelines for Mixing

Preparing a batch of herbicide mixture is not as simple as dumping all the ingredients into a tank. There are general guidelines to keep in mind for proper mixing of herbicides and adjuvants, as well as specific procedures for some of the herbicides mentioned in the previous section.

General Guidelines

It's important to remember that your final mixture volume must include all the herbicide(s) plus adjuvants. If you are making 10 gallons of mix, you cannot put 10 gallons of water in the tank and then add the herbicide and adjuvants, as this would bring the final volume to more than 10 gallons, and the ratio would be incorrect.

However, you also cannot put the herbicides and adjuvants into the tank before adding water, as the chemicals may react with each other to form a substance, such as crystals or "sludge," that would be very difficult to dissolve, again resulting in an improper ratio in solution. Instead, a good rule of thumb is to fill the tank to one-half (50 percent) of the final volume with water, then add the herbicide(s) and adjuvants, and finish filling to the final volume with water. Be sure to agitate the mixture well after the addition of each ingredient to ensure thorough mixing and maximum dilution of each chemical to prevent negative interactions.

Product-specific guidelines
Some product labels will provide additional instructions for specific circumstances, such as mixing that product with certain other products. In an example above, we described a scenario involving a mix of the herbicides glyphosate, triclopyr, and metsulfuron. Glyphosate and triclopyr make a great combination for controlling most plants, but they don’t physically interact well with one another in their concentrated forms. For this reason, glyphosate and triclopyr must be very dilute in order to be successfully mixed for spraying. To mix glyphosate and triclopyr, fill the tank to ¾ (75 percent) of the final volume with water, then add the triclopyr first, mixing it thoroughly with the water before adding the glyphosate. These instructions are typically included in a "Mixing" section on the labels of many glyphosate and triclopyr products. This is another reason why reading product labels is critical.

For proper mixing of dry products, such as the metsulfuron and sulfometuron mentioned earlier, remember that the amount of product is measured by weight. Measure the correct amount using a laboratory scale or the dry product measuring tube that came in the packaging with the herbicide and is specifically designed for that product. If you use a measuring tube, it must be the tube that was packaged with that specific product. Once you’ve measured the correct amount, put it into a cup of hot or warm water and stir until it is dissolved. Then you can pour the cup of water into the mixing tank/container and agitate the mixture well. This process ensures that the dry product is dissolved and does not form clumps that could clog the sprayer or result in an inaccurate mixed ratio.

Summary

Practice spraying in a consistent manner for each calibration application type, paying attention to the visual spray coverage (droplet pattern). Becoming consistent will allow you to calibrate your application rate and then apply at that same rate operationally, so you do not need to recalibrate before every application. It is important to calibrate for each type of application—broadcast, spot, single-seedling plot, or band—as people often apply at different rates for different application types. You may also want to check your application rate by recalibrating from time to time, even if you spray frequently, to ensure accuracy over time.

Knowing your application rate through proper calibration is critical for applying the correct herbicide amounts. In the end, calibrating will save you time, money, and headaches by avoiding repeat treatments, wasted herbicide, or damaged nontarget organisms. Additionally, applicators who master calibration gain a valuable skill and take control of the process rather than simply mimicking instruction that may be incorrect. Calibrating your backpack sprayer requires some time up front, but it is time well spent!

Art Gover and Dave Jackson, reviewers/editors. Other editors include Katie Brooks, Calvin Norman, and Sarah Wurzbacher.