The Need to Screen Varieties for Metribuzin Tolerance

I attended the Tri-State Soybean Forum in Oak Grove, LA last week. One of my primary goals when I attend any of these meetings is to gain new insights into what is needed in soybean research to enhance Midsouth soybean farming tools. I wasn’t disappointed with this year’s meeting in that regard.

During the morning break, Dr. Tom Eubank, a soybean agronomist at the DREC in Stoneville, came up to me and suggested there needs to be a Midsouth/regional screening of soybean varieties for tolerance to metribuzin. He has a great point.

First, a little background on the history of metribuzin, which was sold under the trade names Sencor and Lexone when it first came on the market in the early 1970's.

Metribuzin is a very efficacious residual herbicide, with activity on broadleaf and some annual grass weeds. It also offers a unique herbicide mode-of-action to soybean weed control as a photosystem II inhibitor.

We also found when we first started the Stale Seedbed work at Stoneville in the late 1970's that metribuzin has a high burndown activity when mixed with other herbicides (most of which are no longer on the market). Later, a tank mix of Sencor and 2,4-DB was identified and used as an excellent post-directed spray at layby for control of late-season weeds when we were still growing soybeans in wide rows. Thus, Sencor became one of the more versatile soybean farming tools in the soybean weed control arena.

As great as this herbicide was and still is, it had a significant drawback. Varietal sensitivity to metribuzin was identified, especially when varieties were planted on coarser-textured soils with low organic matter following preemergence application of Sencor, or on the clay soils when the highest rate was used for enhanced cocklebur control.

That precipitated research by Drs. Bill Barrentine and Tom Kilen at Stoneville that resulted in the development of a technique that was effective in classifying metribuzin-tolerant soybean populations. A notable tolerant variety that was released by Drs. Hartwig and Barrentine in 1980 was Tracy-M.

Of course, all of this became mostly moot when glyphosate-resistant (GR) soybeans came on the market in the mid-1990's. The use of residual herbicides such as Sencor fell by the wayside and varietal sensitivity/tolerance to metribuzin was no longer an important consideration for variety selection.

So, for 15+ years, there has been little or no screening for metribuzin tolerance by breeders as they release varieties. Thus, we have no current knowledge of varietal sensitivity to this herbicide.

Now, we are back to square one because of GR weeds; i.e., residual herbicides are again being recommended as a weed management soybean farming tool for soybeans. And guess what. Effective residual herbicides such as Authority MTZ, Boundary, and Canopy that are being recommended contain metribuzin. Thus, Dr. Eubank’s above statement about the need to screen varieties for tolerance to metribuzin is important.

With 200+ varieties available for producers to choose from, it is virtually impossible for any one state to do a screening of all available varieties for metribuzin tolerance. However, if the Midsouthern states get together and split up the varieties, any one state would initially only have to screen 40+ varieties as part of their variety testing program (I didn’t mention it above, but the screening is done in the greenhouse, so information generated by any one state for a particular variety would be usable by all producers regardless of location as long as all screenings use the same technique). The number of varieties to be screened in subsequent years would be much lower since only new varieties that become available each year would have to be screened. Eventually, it is desired that entities that release varieties will incorporate this screening into their breeding programs.

For now, producers need to at least be aware that varietal sensitivity to metribuzin probably still exists, but there is no knowledge of which varieties may be affected.

I encourage the managers of the Midsouth states’ soybean variety testing programs to consider a regional effort to screen for metribuzin sensitivity so that producers will have this knowledge when they select varieties to match their weed management program.

I thank Dr. Tom Eubank for his review and edit of this article.
larryheatherly@bellsouth.net