I just started using Tenacity this summer as we got a puppy and am trying to avoid any chemical herbicides like 24D in his area. The active ingredient is mesotrione, a compound derived from the bottlebrush plant. I think it also has a low risk EPA rating as well. You only need a teaspoon per gallon so your putting down such a tiny amount.
Worked great on my crabgrass. It disrupts photosynthesis and it turns white in a week or two.
It also is acts as a premergent and can be put down with grass seed as well.