Pigweed in soybeans

Soybean residual herbicides: a numbers game?

The University of Arkansas' MP44 adopted the HRAC system of naming herbicide families to make it easier for growers and consultants to quickly determine the modes of action of the herbicides they are using.

In soybean weed control we have been focused on managing the further development of herbicide resistance in Palmer pigweed. Already in Arkansas we have populations of pigweed that are resistant to ALS (Classic/Scepter), DNA (Prowl/Treflan), and glyphosate herbicides. We are currently concerned about developing resistance patterns in some fields to Liberty or glufosinate and the Protoporphyrinogenase herbicides — also known as PPO herbicides — including Valor, FlexStar, Reflex, Blazer and Cobra, among others.

With all the common names and the premixes, the tremendous number of trade names can become confusing and tedious to keep up with. A grower encouraged me to keep using trade names for this very reason — to avoid more confusion. There is an old saying in the ag chemical business: “can ’em and confuse ’em.” That is not my goal as an Extension scientist.

(Click to open MP44)
(Click to open MP44)

In the University of Arkansas MP44, we adopted the HRAC system of naming herbicide families to make it easier for growers and consultants to quickly determine the modes of action of the herbicides they are using. In this system, for example, the ALS family mentioned above is group 2; DNA is group 3; Roundup, 9; Liberty, 10; and the PPOs, 14. More simple. In the weeds controlled tables in the MP44 you will find that the first column of numbers are not control numbers but are herbicide modes of action numbers. Many companies are putting these numbers right on the jug. I vote they should all add MOA numbers on the jug.

For most of the 2000s, we used repeated applications of Roundup for weed control. Roundup burn-down, Roundup post followed by Roundup post. Put another way, we applied a group 9, followed by a group 9, followed by a group 9. See how resistance could develop to all group 9 herbicides?

Glyphosate-resistant pigweed

What concerns me is what is happening in some areas around the country to battle glyphosate-resistant pigweed. One good program/example is Valor followed by Roundup plus FlexStar. This has been used a lot in the Midwest and was used early in our own battle with pigweed in Arkansas — and it is effective. However, to put it another way, we are applying a group 14, followed by a group 9 plus a group 14. Two modes of action right? This is better for resistance right? Well, not really. It might have been if we had been doing it years ago before pigweed became resistant to glyphosate. Now that it’s resistant, in the example above we are basically applying group 14, followed by group 14. See a pattern forming?

We have, in fact, found at least two populations of pigweed in Arkansas showing signs of tolerance, early development of resistance to group 14 or PPO chemistry. Tall waterhemp and other pigweed populations around the Midwest are already resistant to group 14 herbicides. Illinois released the results of a survey showing this just this past year (Tranel and Hagar, 2014).

Luckily the companies have been working overtime to provide a long list of premixes that include multiple modes of action that work on pigweed. Although we are somewhat limited to group 14s for post applications — unless you are growing LibertyLink — there are many residual options. A good example is a product like Boundary (Dual + Sencor), groups 15/4, both groups with activity on pigweed and neither with resistance or relied on post. Authority Elite is Spartan plus Dual or 14/15 and although it contains a 14 it has an equally effective second mode of action. There are others like Authority MTZ and Fierce, which are 14/4 and 14/15, respectively.

We have, in fact, found at least two populations of Other products that contain ALS chemistry and Roundup still have many benefits, but for resistance management you have to be careful which ones you include in your plan. Group 15s (Dual, Warrant, Zidua) can also be applied post and are effective if weeds have not emerged. A popular early post option for example is Prefix, a 14/15, with residual and post activity.

Hopefully the numbers help and do not just add to your confusion.

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