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.
Metribuzin is only moderately persistent in soil, so where it is applied a few weeks or more ahead of planting, increasing the rate or making two applications can improve the longevity of control. The rate used also depends upon whether metribuzin is being applied with other herbicides that have substantial activity on the weeds of interest. So, while we suggest that rates of 0.47 to 0.56 lbs ai/A could be the most effective for residual marestail control in the absence of other residual herbicides, a rate of 0.28 lb ai/A could be adequate to improve the control when applied with Valor XLT, Authority First, Sonic, etc.
The table in the metribuzin label that shows application rates based on soil texture and organic matter content has more detail than similar tables on many other herbicide labels. Metribuzin can occasionally injure soybeans and it is important to follow the rate guidelines on the label to minimize the risk of injury. Injury is more likely where soil pH is 7.5 or higher, and some soybean varieties can be sensitive to metribuzin. Check with the seed company for more information, although many seed companies no longer screen their soybean varieties for metribuzin sensitivity. The risk of injury may be lower than it was 30 years ago when higher rates of atrazine were being used in the preceding year’s corn crop. In addition, applying several weeks or more before planting in no-till probably reduces the risk of injury compared with application at or after planting.