Iowa State University

Are All Metolachlor Products Equal? Version 2.0.
by Bob Hartzler

December 19, 2003 (Updated February 5, 2004 -  Generic products continue to grab an increasing percentage of the herbicide market in agronomic crops.  It has been our position that in most situations a generic product should perform identically as a name brand product since the active ingredients of the products are the same.  Any difference in performance between these products would be associated with the inactive ingredients.  The similarity between the products is reflected in the labels of generic and proprietary products - in most situations the labels are identical. 

Stalwart C and Me-Too-Lachlor are new metolachlor products from Sipcam and Drexel, respectively.   Drexel is selling two versions of Me-Too-Lachlor.  Version 1 does not include a safener, whereas Me-Too-Lachlor II has a safener.  On first glance these products look like any other generic herbicide since use rates for the product are similar to those of Dual II Magnum (Table 1).  However, Stalwart C and M-T-L may represent an exception to the rule of generic equivalency.  The difference lies in the fact that Dual II Magnum contains the resolved isomer of metolachlor (S-metolachlor), whereas the two new products use an unresolved blend of isomers (the form of metolachlor used in Dual formulations prior to Dual Magnum).  The unresolved blend contains forms of metolachlor that are less active than the isomers contained in Dual II Magnum.  Due to the complexity in manufacturing, all metolachlor products (whether Dual II Magnum, Cinch, Stalwart, etc.) contain a blend of four isomers.  Dual II Magnum and other resolved formulations contain approximately 88% of two forms of the more active S-isomers, and approximately 12% of the less active R-isomers.  Unresolved formulations would contain approximately equal ratios of the S and R-isomers.  For simplicity purposes, in this article I will refer to the blend of isomers found in Dual II Magnum (and other resolved isomer formulations) simply as S-metolachlor.

          Figure 1.  Activity of different forms of metolachlor.                       

Ciba published research in the early 1980's documenting the enhanced activity of the S-metolachlor isomer compared to the R isomer and an unresolved blend of the S- and R- isomers (Figure 1).   While R-metolachlor exhibits herbicidal activity, it has less than 50% of the activity of S-metolachlor.  The research documents that the unresolved isomer (combination of S and R- metolachlor) also has less activity than S-metolachlor, but that R-metolachlor does contribute to the activity of this combination.  The difference in activity between the S-isomer and the unresolved isomer was diminished, but not eliminated, at the higher rates evaluated in this study.

When  Dual II Magnum was introduced, they reduced the use rate by approximately 30% to account for the enhanced activity of S-metolachlor (Table 1).  The Stalwart C label recommends the same product use rate as those for Dual II Magnum, but the product contains the less active unresolved form of metolachlor.  This results in Stalwart providing less 'weed-killing power' than Dual II Magnum when applied at equal rates. 

An appropriate question is whether Stalwart C will provide effective weed control at label rates?  It is likely that in a significant percentage of treated fields it will be difficult to observe differences in performance of Dual II Magnum and Stalwart C when the products are applied at the same rate.  Label rates are designed to provide acceptable weed control under a wide variety of conditions, both favorable and adverse (See Apply When Actively Growing).  Thus, Stalwart, and other unresolved formulations of metolachlor, should provide acceptable control when conditions are favorable for herbicide activity.  However, the reduced activity of the unresolved isomer will result in reduced performance in many situations (i.e. heavy weed pressure, difficult to kill weeds, dry weather, late-season weeds, etc.).  

Table 1.  Recommended rates of three metolachlor products for preemergence applications on medium texture soils.

¹Dual II 7.8EC and Stalwart C 7.8 EC contain the unresolved isomer of metolachlor, whereas Dual II Magnum contains S-metolachlor, a resolved isomer with enhanced activity.

There are a number of products available that utilize metolachlor as the active ingredient, thus it is important to know which form is present.  This can be determined by examining the ingredient statement found on the first page of the label.  If the active ingredient is listed simply as metolachlor, then the product is a mixure of isomers similar to Stalwart.  If the active ingredient is listed as S-metolachlor, then the product primarily contains the active isomer used in Dual II Magnum.  When evaluating whether Stalwart represents a good value for your farm, consider both the cost and the relative performance you can expect from it compared to Dual II Magnum.  If Stalwart costs significantly less than resolved isomer formulations, then it may represent a reasonable value when used under appropriate conditions.

Stalwart Xtra is an atrazine premix that will be similar to Bicep II Magnum, with a similar reduction in metolachlor activity.

Related articles
    Resolved Isomers Explained

Acknowledgements:  I appreciate the information provided by Jon Gehring, Sipcam, regarding the composition and activity of the various metolachlor isomers.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University