Farm Progress

If you don’t know how many sites of action your herbicide program attacks, you may be sacrificing weed control and helping develop further resistance.

Tom J Bechman 1, Editor, Indiana Prairie Farmer

January 19, 2018

3 Min Read
KNOW SITE OF ACTION: The stakes are too high to leave weed control to guessing. Jeff Nagel recommends figuring out how many sites of action your herbicide program covers.

Jeff Nagel talks about herbicide sites of action like most agronomists talked about herbicide brand names just a few years ago. Nagel, with Ceres Solutions Cooperative, based near Lafayette, Ind., says it’s not enough just to know which brand names or even which active ingredients you’re applying today. You need to know which sites of action each herbicide attacks to kill target weeds.

Why? Because many weeds have already developed resistance to whole classes of herbicides, he says. Some — like waterhemp, which is moving eastward into Indiana from Western states — are already resistant to several different classes of herbicides.

If you don’t know which sites of action each herbicide attacks, you could be applying three or four herbicides, but only hitting one site of action, Nagle says. If weeds like waterhemp are already resistant, you won’t get control. If they’re not yet resistant, applying herbicides with only one site of action year after year will help weeds develop resistance faster.

Herbicide exercise
Working with WinField United, Ceres Solutions developed a simple exercise to help farmers determine how many sites of action they’re applying. If you’re not applying enough, use the website to design a herbicide program that includes more sites of action.

You can use the worksheets to compare herbicide programs for any target troublesome weed in your area. Waterhemp is used as the example here, but you could do the same exercise for giant ragweed, marestail, Palmer amaranth and a number of other weeds, Nagel says.

Example 1: Existing soybean program. Suppose your current program for soybean weed control is applying 3 ounces per acre of Valor XLT preemergence, followed by 32 ounces per acre of Roundup PowerMax and 1.25 pints per acre of Flexstar.

Valor XLT works on two sites of action, Nagel notes — sites affected by herbicide classes 14 and 2. PowerMax works on one: Class 9. Flexstar is also Class 14.

You applied herbicides that work on three sites of action, he says. However, if the waterhemp in your field is already resistant to Class 2 (ALS inhibitors) and Class 9 (glyphosate), you only applied one effective site of action! That’s Class 14 (PPO inhibitors).

Example 2: Possible program for dicamba-tolerant soybeans. Here is one alternative program among several options you could try, Nagel says.

Start with Zidua Pro at 6 ounces per acre and Dimetric, which is metribuzin, at 5 ounces per acre. Zidua Pro contains three active ingredients with three sites of action: 2 (Pursuit), 14 (Sharpen) and 15 (Zidua). Only the Zidua in Zidua Pro provides an effective site of action, since most waterhemp is resistant to Class 2, and the Sharpen rate is too low for residual control. Metribuzin is Class 5.

Come back postemergence with Roundup PowerMax at 32 ounces per acre (Class 9), plus Engenia at 12.8 ounces per acre and Zidua at 1.5 pounces per acre. Engenia contains dicamba, Class 4. Zidua covers Class 15 again, but would be used for in-crop residual control.

The program covers six sites of action. Three sites of action are still effective against waterhemp: 15 (Zidua), 5 (metribuzin) and 4 (Engenia).

Why include herbicides from classes resistant to waterhemp? “You’ve got other broadleaves and grasses to control, and they may still be effective against them,” Nagel says.

Nagel recommends doing the same exercise for your corn herbicide program.

About the Author(s)

Tom J Bechman 1

Editor, Indiana Prairie Farmer

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