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.

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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.