Editor’s note: This is Part 3 of a four-part series on using multiple, effective herbicide sites of action at effective rates as part of a long-term weed management system. 

By Meaghan Anderson and Bob Hartzler

It is important to use multiple, effective herbicide groups.

After we’ve passed those tests, it is equally important to be sure we’re using an effective application rate to manage the 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 preemergence products commonly are applied to reduce costs or reduce the risk of crop injury or herbicide carryover. Although 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 of weeds exposed to the postemergence program, which increases the risk for herbicide resistance to develop with those products. The shorter residual also reduces the window for timely application of the postemergence product.

Evaluate application rates
To evaluate the rates of your herbicides, 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. HG 2 is not effective against waterhemp because of resistance, so the product only contains one effective herbicide for waterhemp management.

What if you 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, you need to know the application rate of each product and the concentration of the chemical in the products. This requires that you know what the herbicide label says. Visit 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, you can find the information on product concentration necessary to do the math to compare rates. Most labels will have the herbicide group numbers on the first page, as well.

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 pound of 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.

Next, you need to find the application rate of the product to determine how much active ingredient is applied per acre. 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.

Assume the soil has greater than 3% organic matter and has 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 pound sulfentrazone per acre. 

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

Again, the herbicide label contains the herbicide group number for the product and the concentration of the active ingredient directly below the list containing the active ingredient.

The concentration is noted in pounds of active ingredient per gallon since it is a liquid product.

Again, assume the soil has more than 1.5% but less than 3% organic matter and is a medium to fine texture. You just need to make sure you are comparing rates of the two products (Authority First and Spartan) for the same soil type. You 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. Lay the math out as shown below. You first convert the fluid ounces to gallons since the concentration is in pounds per gallon.

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

Do math yourself
What does this mean for your 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 they reduce the rate in order to reduce costs.

How much of a reduction in the applied rate is acceptable is subjective, but 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 yourself. You then will be more aware of the expected period of weed control with a preemergence herbicide program and be 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 application rate — if the label allows.

Hartzler and Anderson are Iowa State University Extension weed control specialists.