Herbicide program development: Herbicide rate calculations

December 11, 2018 12:47 PM
Blog Post

This is part three of a four-part series originally posted in 2016 on using multiple, effective herbicide sites of action (herbicide groups) at effective rates as part of a long-term weed management system. Read part one and part two now.

It is important to use multiple, effective herbicide groups, as discussed in the previous two parts of this series. After we’ve passed those tests, it is equally important to be sure we’re using an effective rate to manage our weeds. Many herbicide products combine multiple herbicides and herbicide groups to create effective products for yield protection and ease of use. Sometimes these premixes, even when used at full label rates, will use lower rates of individual products compared to the stand alone product. 

Reduced rates of premergence products are commonly applied in order to reduce costs or reduce the risk of crop injury or herbicide carryover. While reduced rates can be effective at preventing early-season competition by controlling the first flush of weeds, they greatly reduce the length of weed control provided by the preemergence herbicide. The shorter residual control results in a larger population exposed to the postemergence program, and increases the risk for herbicide resistance to those products. The shorter residual also reduces the window for timely application of the postemergence product.

In order to evaluate the rates of your herbicides, we must compare those premixes and package products to the full rate of individual products containing only one active ingredient.  For example, you might choose to use the product Authority First for preemergence waterhemp control. This product contains two herbicide groups, 2 and 14. We know that HG 2 is not effective against waterhemp due to resistance, so my product only contains one effective herbicide for waterhemp management. I want to compare that HG 14 product, sulfentrazone, in Authority First to the full rate of sulfentrazone in a product that only contains sulfentrazone, Spartan.

When comparing two products, I need to know the application rate of each product and the concentration of the chemical in the products. This requires that I know what the herbicide label says. I like to use the resource www.cdms.net to look up product labels. The directions to find herbicide labels on this website are available in Part 1 of this series

On the first page of the herbicide label, I can find the information regarding product concentration necessary to do the math to compare rates. Most labels will have the herbicide group numbers on the first page as well.

Notice the herbicide groups and active ingredient concentrations

Authority First contains 0.62 lb sulfentrazone in every lb of product

The concentration of individual active ingredients is directly under the active ingredient information on the first page of a herbicide label.  You can see the concentration of sulfentrazone is 0.62 pounds active ingredient per pound of product.  This tells you it is a dry product.  If the product was liquid, it would be given in pounds of active ingredient per gallon of product.

This label does the math for us

Next, we need to find the application rate of the product to determine how much active ingredient is applied per acre (picture above). Consider the characteristics of your own soils when determining the full herbicide label rate. Also, be sure to use the same conditions when comparing across the two labels. I am assuming the soil will have < 3% organic matter and will be a medium-fine texture. The appropriate full label rate for those conditions would be 6.45 dry ounces per acre. This label has already calculated the amount of active ingredient applied per acre: 0.25 lb sulfentrazone per acre. 

Next, I need to find the herbicide with sulfentrazone as the only active ingredient to compare this rate to. That product is Spartan.

Notice the herbicide group and active ingredient concentration

Again, you can see this herbicide label is set up similarly to the previous one. It contains the herbicide group number for the product and the concentration of the active ingredient directly below the list containing the active ingredient.

Spartan contains 4 lb sulfentrazone in every gallon of product

You’ll see this concentration is noted in pounds of active ingredient per gallon since it is a liquid product.

This label provides me a rate range, but I have to do the math.

Again, I’m going to assume my soil has more than 1.5% but less than 3% organic matter and is a medium to fine texture. I just need to make sure I am comparing rates of the two products (Authority First and Spartan) for the same soil type. I can apply 10.1 fluid ounces of product per acre. How much sulfentrazone is in those 10.1 fluid ounces? 

Time to break out the pencil, paper, and calculator. I like to lay the math out as shown below. I first convert the fluid ounces to gallons, since the concentration is in pounds per gallon.

Table of rate calculations

A full rate of Spartan has 0.316 lbs of the active ingredient sulfentrazone, while a full rate of the premix, Authority First, contains 0.25 lbs of the active ingredient sulfentrazone.  Authority First has about 79% of a full rate of sulfentrazone.

What does this mean for my preemergence herbicide application? Ideally, the rates applied in premix products should be the same as the single active product. However, manufacturers frequently reduce the rate assuming there will be additive action among the active ingredients or in order to reduce costs. How much of a reduction in the applied rate is ‘acceptable’ is subjective, but I would like to use at least 75% of a full rate. In this example, the second active ingredient in Authority First will not help in controlling waterhemp, so you are relying solely on the reduced rate of sulfentrazone to control this weed. 

It’s important to do this math yourselves. You will then be more aware of the expected period of weed control with a preemergence herbicide program and able to make better management decisions based upon that knowledge. You also may have the ability to ‘spike’ your preemergence herbicide application with more active ingredient to reach that 100% full rate, if the label allows.


Meaghan Anderson Field Agronomist in Central Iowa

Meaghan Anderson is a field agronomist in central Iowa and an extension field specialist at Iowa State University Extension and Outreach. Educational programming is available for farmers, agribusinesses, pesticide applicators, certified crop advisors, and other individuals interested in...

Bob Hartzler Professor of Agronomy

Dr. Bob Hartzler is a professor of agronomy and an extension weed specialist. He conducts research on weed biology and how it impacts the efficacy of weed management programs in corn and soybean. Dr. Hartzler also teaches undergraduate classes in weed science and weed identificatio...