Preemergence mesotrione use in “mesotrione-tolerant” soybeans

By Dr. Mark Loux, OSU Extension Weed Specialist

Put this one in the “we’re usually the last to know” category.  Following an article in the last C.O.R.N. about the Alite 27 label for use on GT27 soybeans, we became aware that some mesotrione products are labeled for preemergence use on “mesotrione-tolerant” (GT27) soybeans.  Products that we know about with this label include Bellum, Motif, and Meso Star.  As far we know, all GT27 soybeans are tolerant of mesotrione used preemergence.  The catch is that the seed tag and agreement need to specify that the variety is “mesotrione-tolerant” for this to be a legal application.  At least this is how it was explained to us by one reputable company rep.  Not every company selling GT27 seed has made this change, so check with seed supplier if in doubt.  Basics of this label are as follows:

– use prior to soybean emergence only and only on soybeans labeled “mesotrione-tolerant”

– use rate of up to 6 oz product/A; only one application (higher rates improve length of residual and improve control of giant ragweed and other tough weeds)

– can be mixed with other preemergence soybean herbicides unless prohibited on another label

– for control of emerged weeds, add AMS plus either NIS or COC (would also depend upon what else is in the mix for burndown)

– do not apply to emerged soybeans

– do not graze or feed soybean forage or hay to livestock

Mesotrione and Alite 27 (isoxaflutole) are both group 27 herbicides (HPPD inhibitors) with similar residual activity on broadleaf weeds.  Alite 27 has considerable residual activity on grass weeds also, while mesotrione doesn’t.  But mesotrione controls emerged weeds, while Alite 27 largely doesn’t.  Given that it’s used on the soybeans with the right seed tag, mesotrione has a state-wide label, while Alite 27 is labeled in certain counties only.  Our thinking on where mesotrione fits amid a plethora of residual soybean premix products:

– can be a lot of broadleaf control for the money, depending upon price.

– can add 3 to 6 ounces to other residual herbicides to improve control of waterhemp, giant ragweed, common ragweed, cocklebur, morningglory, burcucumber.  Or add reduced rates of other residual herbicides to mesotrione.

– Alite 27 and mesotrione are the only two herbicides that provide residual control of giant ragweed populations with resistance to group 2 herbicides (ALS inhibitors).  They also control common ragweed with group 2 resistance, for which flumioxazin is the only other option (and it’s only fair to good).

– can improve control of emerged weeds in no-till situations, when mixed with other burndown herbicides.  Could be especially useful for this where glyphosate is scarce.

– the ratings for mesotrione effectiveness in the corn section of the weed control guide should be accurate for use in soybeans also.

Here’s the potential pitfall with using mesotrione or Alite 27 in soybeans.  We use a lot of mesotrione and isoxaflutole,

and other group 27 herbicides, in corn.  Using them in soybeans also is likely to increase the rate of development of weed populations with resistance to this site of action.  Resistance to group 27 herbicides has already occurred in waterhemp in states west of us, and results of our screening of Ohio waterhemp populations indicates it’s probably occurring here also.  While rotating herbicide site of action from year to year does not on its own prevent resistance, it’s one of the recommended practices to slow down resistance.  Along with mixing multiple sites of action together.  Choosing to use the same herbicides in corn and soybeans, which we already do to some extent, just reinforces the importance of scouting fields in late season to prevent escapes from going to seed.

Note:  this article represents the true story as far as we knew on March 14, 2022