Issue 4, June 20, 2023

Optimizing the Activity of Tenacity Herbicide

Tenacity (active ingredient mesotrione) is a herbicide that can control a variety of weeds in cool-season turfgrasses. Most herbicides used in turf fall into two categories, either postemergence (which means the herbicide is applied to emerged, actively growing weeds) broadleaf herbicides, (e.g. Trimec), or preemergence (which means the herbicide must be applied prior to weed germination and emergence) grass herbicides, (e.g. Dimension or Barricade). Tenacity doesn’t neatly fit into either of those two categories, which makes a valuable herbicide for some difficult to control turf weeds. The label states that Tenacity has both preemergence and postemergence activity. And while this is technically true, the preemergence activity in turf is relatively short-lived and not economically or agronomically valuable. I believe this is in part because a turf environment is conducive to more rapid breakdown of herbicides as compared to when the herbicide is applied to bare soil as when used in corn.

Crabgrass bleached white in response to a mesotrione herbicide application, Travis Cleveland, University of Illinois.

Tenacity is a good postemergence herbicide that can control a number of broadleaf, and importantly, grass weeds. Tenacity can control several perennial grass weeds that are very difficult or not possible to control with other herbicides, including creeping bentgrass (Agrostis stolonifera), nimblewill (Muhlenbergia scherberi), and annual bluegrass (Poa annua). Tenacity provides postemergence control of a number of broadleaf weeds such as dandelion, white clover, ground ivy, etc. and postemergence control of annual grass weeds such as crabgrass, yellow foxtail, and barnyardgrass. This is a versatile herbicide, but the steps needed to optimize the activity, and get maximum control are not commonly practiced.

We’ve found two factors that can increase the activity of Tenacity. First is the addition of either urea ammonium nitrate (UAN) or urea to the spray solution. If you look at the Ag label for mesotrione, sold under the trade name Callisto, you’ll see that they recommend adding either UAN or ammonium sulfate (AMS) for all postemergence applications. For some reason, this recommendation is omitted from the Tenacity label. The amount of nitrogen applied with Tenacity or Callisto is small, usually less than 0.1 lbs N/M, the effect is to increase the activity of mesotrione although the mechanism is unclear.

Second, and more important, is to understand that mesotrione can be absorbed by both the foliage and the roots of weeds. Foliar absorption is often sufficient to control many weeds and particularly broadleaf weeds. However, for grasses, where the meristem, or crown, is at the base of the plant, we often see that foliar applications will bleach the exposed leaves, but will not reach the meristem in sufficient quantity to actually kill the plant. These plants will turn white and look like they are dying, but then will recover. The reason for this response is a lack of absorption of mesotrione by the grass roots. My opinion is that variable control with mesotrione observed by many turf managers is simply due to whether or not the turf received rain or irrigation within 7-10 days of a mesotrione application.

As an example, let’s say you’ve decided to try Tenacity to control emerged crabgrass in lawns. Five days after the second application (because the label indicates you need two applications to control crabgrass postemergence) the site receives 0.75” of rain. A week later you notice the crabgrass is gone, 100% control! You’re quite happy and you decide that this is the best product for postemergence crabgrass control. But then next year you use Tenacity just like you did the year before, but no rain occurs and you see only 50-60% crabgrass control. In year 2, you’re unhappy and are thinking of switching to another product. That is the difference between foliar plus root activity (year 1) and foliar-only activity (year 2). The kicker here is that mesotrione is pretty strongly adsorbed to organic matter so it takes a lot of water to move some into the root zone. Applying a quarter inch of irrigation may seem like a lot but may not be enough to get significant root uptake. So clearly best control will be had when the site receives a significant rain event following application. If rain doesn’t come, then by all means irrigate, but irrigate more than you normally would to attempt to push as much mesotrione as you can into the rootzone.

Bruce Branham

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