Professional Equivalency Constants for Herbicides

By Gil Medeiros, Fairfax Master Gardener
Has this ever happened to you? You purchase a selective herbicide in concentrated form. You mix it in a sprayer exactly according to the instructions on the container. Carefully following the instructions, you apply the chemicals. The next day you inspect your work. Nothing has happened. You have an odd sense the weeds are laughing at you in their own smug, weedy way. Hoping these chemicals work more slowly than expected, you check the next day, and the next. The clover and ground ivy look healthier than ever. It is clear your spray had no effect. What happened?

Most likely, the chemicals were too dilute. “How could that be?” you ask. “I followed the directions as if they were handed down on graven tablets.”

What you don’t know is that the chemical companies believe that average homeowners like you and me typically apply too much herbicide. Our excess chemicals run off into the environment, particularly streams and waterways where, combined with the run-off from our neighbors, they cause a pollution problem. Another ill effect of overuse is that we may kill the grass we are attempting to save from the weeds. Chemical companies do not like reviews on the Amazon website or elsewhere that say their stuff killed Harry’s lawn. Negative publicity is bad for business.

So the chemical companies give us instructions that are intended to remedy our overuse tendencies. However, the resulting dilution is too weak to kill some of the weeds in our lawns unless we apply what seems to be an extreme excess of chemicals … as the chemical companies expect us to do.

Here is a secret. The chemical companies provide different instructions on chemical containers sold to lawn care professionals … same chemicals, but different instructions for professional users. The operating assumption is that the pros have training and experience that will prevent overuse of the chemicals. Therefore, the pros are trusted with more effective dilutions.

But you are a conscientious user. What are you to do?

The Virginia Cooperative Extension, courtesy of Dr. Shawn Askew, has provided you with a way to use the dilutions the pros use. If you consult the Virginia Pest Management Guide (PMG), Section 5 on lawns, you will find something called Professional Equivalency Constants. VCE provides a simple formula and four tables containing Professional Equivalency Constants for different herbicide products. Just do the calculations, and you can use the chemicals the way the pros do.

I tried it, and it worked. I applied Chickweed, Clover and Oxalis Killer by Bonide in the spring according to label instructions. The next day the chickweed and clover looked only slightly wilted. Two days later they had bounced back to full health. It was time to try the VCE calculation. Waiting 21 days, per label instructions, I mixed a new batch of spray according to the formula provided by VCE.

Here is how to do it. First, you will need to find the active ingredients and their concentrations in the product. You may need a magnifier to read information on some of these containers.

For this product they were: MCPA 13.72%
Triclopyr 1.56%
Dicamba 1.35%

Add up these numbers: 16.63%, which is A in the formula below

Then you will need to find the volume of the product container. In this case it was 16 ounces. In the formula this is B.

And find the area, in thousands of square feet, that the entire container is intended to treat. In this case it was 5000 square feet. Divide that by 1000 = 5. In the formula this is C.

Next you must find the Professional Equivalency Constant for this product. See Table 5.8 in the PMG. Search the far left column for products with the same active ingredients as the one you will use. The 13th one down contains Triclopyr, Dicamba, and MCPA. The concentrations in the third column are exactly the same as listed on my container, so this is my product. The Professional Equivalency Constant from the fourth column is 83.2.

The formula is shown below. Plug in the numbers, and don’t worry about units.
formula 1

So this means that using the dilution instructions on the container sold to homeowners, we are using only 64% of the active ingredients used by professionals. You have one more calculation to do.

The standard label instructions for this product call for using 3.2 ounces of concentrate in 2.5 gallons of water. I have a 2.5 gallon sprayer so I now need to calculate how much of the product to put in the sprayer (assuming I will fill the sprayer to 2.5 gallons) according to my professional equivalency calculation.

formula 2

I mixed the product and applied it according to the instructions.

Eureka! This time it worked. The weeds were controlled as expected.

Make sure you read the fine print provided in the PMG. You don’t want to violate federal law in the application of the chemicals.

If you don’t like to mess with equations, but you have Excel on your computer, I have provided a spreadsheet to automate these calculations.
Spreadsheet for Professional Equivalency calculations

Give it a try!

Pest Management Guide: Home Grounds and Animals, 2016, Virginia Cooperative Extension, 456-018