Herbicide Calibration for Roadside Applications

Jan 24, 2024

Roadsides are often a very difficult place to control weeds. Weeds thrive on roadsides and roadsides and rights of way are obviously long and narrow. Weeds along the edge of the shoulder need to be cleared to reduce fuels to prevent wildfires, and to ensure safety for drivers and also so that weeds do not damage the road surface or shoulder structure.

Herbicide applications on roadsides are often conducted at large scales where several acres to many acres are treated in a day. Roadside equipment can also be highly specialized ensuring large stretches of public roads and adjacent lands are better protected.

Herbicide applications on roadsides should be calibrated to ensure the amount of herbicide applied is the amount that was intended in the recommendation. There are generally two ways to ensure roadside herbicide applications are properly calibrated. First is the 128th acre method the second is the 495 method. The 128th acre method is best for small to medium sized applications, such as with backpacks, spray guns or small booms. The 495 method is most useful for fixed booms and vehicles traveling at a constant speed. I'll briefly describe both here, but also note this blog is not a comprehensive guide.

128th acre method (see attached worksheet)

In the 128th acre method we use the fact that there are 128 ounces in a gallon and if one sprays a 128th of an acre (340 sq. ft.), then the two 128's cancel out and the amount of water applied to the site is the gallons per acre. The amount of water used to apply the herbicide (in gallons per acre, GPA) is nearly the total volume of liquid used in an herbicide application.

In the 128th method before starting, the applicator should work with clean equipment and triple rinse any tanks, hoses, backpacks or other equipment. Then the applicator measures out a 128th acre parcel on the ground, 10 x 34 ft. or 18.5 x 18.5 ft. are two common measures. The applicator then sprays the 128th acre while a second person times how long it takes to spray the 128th acre with water. Let's say it takes 30 seconds. Next the applicator sprays for the same time (with their partners help timing again) into a measuring bucket (again for 30 seconds in our hypothetical example). The amount of water in the bucket at the end of the time is the GPA (let's say it was 30 secs. = 40 ounces = 40 GPA).

Now we get to the math part, aka the trick. The GPA just tells us how much mix we would have applied to the site, not how much herbicide to add to the tank. To find how much herbicide we add to the tank we take the size of our spray tank and divide by the GPA. Lets' say our tank is 60 gallons:

60 gallon tank / 40 GPA = 1.5 acres per tank.

Now second half of the trick is to take this acres per tank and multiply by the amount of herbicide in the recommendation. Let's say our recommended herbicide rate is 20 ounces per acre:

1.5 acres per tank  x  20 ounces per acre = 30 ounces of herbicide per tank.

These calculations are in the 128th acre worksheet on this blog page, in the middle of the worksheet. To check if this makes sense, since our 60-gallon tank will spray 1.5 acres, we should have 1.5 times larger amount of herbicide than the recommendation for our final answer (which is 30 ounces). The worksheet that I have attached should help guide you through this calibration process including both calculating your GPA and then using the trick to determine the herbicide to add to the tank. (Note of caution: We are currently upgrading this worksheet and should have a new version out in a year).

495 method

This method is similar to the 128th acre method in that we determine our GPA and then use the math trick to calculate out the amount of herbicide to add to the tank. This method works best when driving at a fixed speed and using a fixed spray swath, which often happens when spraying with roadside trucks.

To calculate the GPA we use the equation:

(495 x flow rate (in gal per min. GPM) ) / (Speed (in MPH) x Swath width (in ft.))

Which equals the GPA

(note: multiply the 495 and GPM, and the speed and swath separately, then divide)

To calculate flow rate, for most roadside equipment there will be a computer and flow meter on an output line to measure flow rate (in volume per minute). If you do not have a flow meter on your setup then you'll need to put buckets or hoses and buckets under each nozzle and spray for set amount of time (30 seconds or 1 minute work well) and record the volume of water in the measuring bucket (in gallons per minute, there are 128 ounces per gallon). If using multiple nozzles on a boom, add all the buckets or nozzles together. Speed will be determined by driving speed (speedometer) and swath width can be measured by spraying with water in place for 15 seconds and then measuring out the wet pattern on the ground.

Next, we can calculate the GPA. Let's say we spray at 3 GPM, drive at 5 MPH and our width is 5 feet.

(495 x 3 GPM ) / (5 MPH x 5 ft ) = 1,485 / 25 = 59.4 GPA (let's round up to 60 GPA).

Now we do the same trick as we did with the 128th acre method and take our tank size and divide by GPA to get acres per tank (let's say this time we have a 600 gallon tank)

600 gallon tank / 60 GPA = 10 acres per tank.

Now we repeat the second part of the trick and multiply our acres per tank by herbicide rate in the recommendation (let's say its again 20 ozs. per acre). We get:

10 acres per tank  x  20 ozs. herbicide per acre = 200 ozs. herbicide per tank

Both methods rely on determining the spray GPA and then using your equipment size to determine the amount of herbicide to add to the tank. Following these steps will help ensure you are not over applying or under applying herbicides on roadside settings.