As with most planting seasons, this one has been anything but normal. Cool conditions delayed the emergence of Palmer pigweed, but high densities are now being observed in many fields across Arkansas. As a result, it is imperative that we start clean at planting and begin with a good residual herbicide.

The number of pre-emergence residual options in soybean is vast, but obviously not all pre-emergence herbicides provide comparable levels of control. In trials I’ve conducted over the past several years, one herbicide consistently ranks at the top of the list for residual Palmer pigweed control and that is a full rate of metribuzin.

Metribuzin was once a major herbicide in soybean, but some soybean varieties are somewhat sensitive to metribuzin – meaning you need to grow a tolerant variety if you plan to apply metribuzin. Prior to Roundup Ready soybean and Roundup-only weed control programs taking over the soybean industry, Dick Oliver, weed scientist at the University of Arkansas, screened Mid-South soybean varieties for tolerance to metribuzin.

In 2012, seeing the renewed need for metribuzin in soybean, I chose to reinitiate the metribuzin screening program at the University of Arkansas. The degree of tolerance of 310 soybean varieties was established last year and this information is available at http://bit.ly/14R4cTR. Again this year, I will be screening all varieties placed in the Arkansas soybean variety testing trial along with a few additional varieties.

We are not using enough metribuzin. In soybeans, we are placing extreme selection pressure on our PPO (Group 14) herbicides – similarly in cotton. In the Midwest, waterhemp, a close relative of Palmer pigweed with resistance to the PPO herbicides is beginning to be found more frequently.

As of today, I’m not aware of any PPO-resistant Palmer pigweed, but it is likely the day is coming soon when we will see resistance to this group of herbicides. Increased use of metribuzin would most definitely help protect the PPO herbicides.

PPO herbicides for soybeans

In soybeans, PPO herbicides that are being applied at planting are Reflex and Valor. Additionally, there are a lot of premix products available for at-planting residual weed control in soybean that contain a PPO herbicide. Some that come to mind are Authority XL, Authority MTZ, Prefix, Fierce, Verdict, Envive, and Broadaxe.

I routinely talk about the need for using multiple effective modes of action against pigweed or any other resistant-prone weed as a means of protecting herbicides against resistance. Many premixes are touted as an excellent tool for resistance management because multiple modes of actions are comprised within the premix.

Unfortunately, many of these are recommended at rates that fall far short of the use rate recommended for each active ingredient alone. There are likely three reasons for recommending a lower rate when applying two products together.

First, if Palmer pigweed is one of the weeds we are targeting with the premix and both of the components alone are effective on pigweed, an application can still achieve effective control by reducing the rate of each of the components.

Herein lies a problem. Now we have a reduced rate of each component and there is a wealth of recent research showing that reduced rates of herbicides contribute to resistance.

Second, reducing the rate of the combination is one means of allowing a company to remain competitive while offering “multiple” herbicides in a premix.

Third, for some herbicides such as metribuzin, it is my belief that use of this herbicide in a premix is often recommended at a low rate to ensure that there are no tolerance issues when applying the product in soybean.

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The problem here is that the metribuzin component is lowered to the point that a sensitive soybean variety will not be injured by the metribuzin, but in turn the rate is so low that the metribuzin provides marginal pigweed control. I actually took a look at several products this past summer and when I applied metribuzin alone at the low rate contained within the premix, the result was utter failure on pigweed.

Again, metribuzin is an excellent herbicide that needs to be on more acres of soybean. If you are over 40 years old, you most likely remember metribuzin as Sencor, but today there is no Sencor available for use in soybean – it is only registered for use in turf. Today, straight-run metribuzin is sold for use in soybeans by Mana, Loveland, and UPI under the trade names of Tricor and Metribuzin.

Next time you plant soybeans, consider applying a full rate of metribuzin in areas where pigweed has been a major problem and don’t forget to plant a tolerant soybean variety. I think you will be impressed with the level of pigweed control.

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