Revisiting metribuzin: It’s back with more utility
Several years ago a dealer commented to me that he didn’t know why he would ever need to recommend metribuzin to a grower. I remember thinking that there was a need somewhere for almost every residual herbicide available to us, and that some would likely become more important due to herbicide resistance issues. We seem to have arrived at that point with metribuzin. This is of course not news to those readers who never stopped using it, but for the rest of us, this article hopefully provides the reason to use or recommend metribuzin in some soybean fields.
Metribuzin is a triazine herbicide and photosynthetic inhibitor that has been used in soybeans since the mid 1970’s, and it was a primary component of soybean herbicide programs prior to the introduction of ALS-inhibiting herbicides in the late 1980’s. Sold by Bayer under the name Sencor for several decades (but no longer), generic metribuzin is currently available from several companies (e.g. Metri DF, Tricor, Dimetric). Metribuzin is a component of several premix products also, including Canopy/Cloak DF, Boundary, Matador, Intimidator and Authority MTZ.
Metribuzin is one of the few residual herbicides that also has substantial burndown activity, primarily through non-systemic activity on small annual weeds. While this activity by itself is usually not adequate for control of emerged weeds, combining metribuzin with other burndown herbicides can improve the overall effectiveness of the burndown. Metribuzin’s contribution is maximized by mixing it with other burndown herbicides that work via contact activity (non-systemic), such as Liberty, Gramoxone, and Sharpen. This can be an advantage that metribuzin has over the two other herbicides that are also widely used for residual control of marestail, flumioxazin (Valor) and sulfentrazone (Authority/Spartan), which do not provide any control of emerged weeds and have some potential to antagonize the activity of systemic herbicides.
The spectrum of residual control for metribuzin includes most small-seeded annual broadleaf weeds. It is most effective on lambsquarters, pigweeds, Pennsylvania smartweed, ladysthumb, marestail, and waterhemp, but also has some activity on common ragweed, velvetleaf, and annual grasses. Metribuzin does not control weeds that are resistant to triazine herbicides of course. We consider the rate of 0.28 lb ai/A (6 oz of 75DF) to be about the minimum for effective control of any annual weeds, and we have observed more effective control with increasing rate between 0.28 and 0.56 lbs/A. Where the goal is to attain a certain metribuzin rate by combining it with another premix that also contains metribuzin, it’s obviously important to know how much metribuzin the other product contains. This information can be found in Table 18 of the “2013 Weed Control Guide for Ohio and Indiana”. For example, the 4 oz rate of Canopy DF contains 0.16 lb ai of metribuzin, so reaching a metribuzin rate of 0.47 lbs ai would require the addition of another 0.31 lbs, or 6.6 oz of a metribuzin 75DF product.