Tenacity (mesotrione) is used to kill bentgrass, clover, and other non-desirables. I get how an herbicide could selectively kill broadleaf weeds while leaving grasses alone, but how does it manage to kill bentgrass and poa annua while not harming other turf grasses?
That particular herbicide is known as a HPPD inhibitor. It works by blocking an enzyme in the plants. The plants normally break down a particular amino acid (tyrosine) and use the components to build other needed substances. I'm guessing that normal lawn grasses get their nutrition via a different method, so blocking that enzyme doesn't effect them.
I checked on this herbicide and it's main use is for pre-emergence of grass seeds. Guess it must work but lawns shouldn't get to the point where you are able to see the soil. Haven't read the details for target species but pre-emergent chemicals can leach (move to other areas and into the water table), inhibit growth of other seeds in the garden and hamper growth of annuals and other young plants and then end up in our water.
The best way to not need this chemical is to grow dense, thick grass. Minimum of 3" in height. Then sunlight is not able to reach grass seeds/weed grass seeds such as Poa annua. Poa annua is very light colored and seems to always have seeds. That grass is what many golf courses use as well as the bent grass. One bird eats a seed, poops it out on a nice green lawn other than the golf course crop, if that lawn is too short then that seed will germinate.
Do you live near a golf course? Extremely important to understand how to promote your grass and not promote these shallow rooted golf course weed grasses. Keep your lawn thick so no sun gets to the soil and odd weed seed.
The best way to encourage the lawn grasses you want and discourage the weed grasses is by watering. Water only when the lawn bed is dry! When you walk across the lawn, your footprints will still be visible. This is the best signal that your grass needs water. This trains our genetically predisposed cool season grasses for the North West with huge root systems to grow deeper in an effort to reach further into the soil profile for moisture. This makes your lawn drought tolerant. Any little drought (like a day or two of no water) shallow rooted grasses will die out quickly and have little chance for seeds to germinate. While your lawn is thriving on once per week watering.
Look up the MSDS sheets for this product and it will explain the chemical in far more detail. I am just trying to explain how NOT to have to use any chemicals while having the best lawn you could imagine. Chemicals are band aids. In the real world if lawns were at all natural we wouldn't have to even add fertilizer or water. If it comes down to using chemicals then something is wrong.
Mow no lower than 3". Fertilize when necessary and use extended release fertilizer. Bag your clippings. Mulching is a nice idea but still not doable. Sharp sharp blades. What do you do for watering, for any chemicals added, for pH (lime addition)? For clippings...anything you can add we will be able to get your practices correct so you won't have to put band aids on problems produced by maintenance practices...a healthy lawn will never need any chemicals ever except of course fertilizer, possibly lime. That is it. Anything else is just a band aid for problems that can be prevented. I read the Tenacity brochure and trust me it is made for the company's benefit. No one needs this product. Healthy lawn by basic healthy maintenance practice is POSSIBLE! Get a soil test while you are focused with your lawn. Cheap, call your nearest Cooperative Extension Service of your main University. Without a test, never add lime.