I've never used mesotrione, but the Tenacity label seems to indicate that it will not effectively control mature Poa annua as a post-emergent application. Look under the section labeled "Weeds Controlled".

I have had some success spot spraying triclopyr + fenoxaprop tank mixed to control creeping bentgrass in my KBG lawn. It will probably take 2-3 applications but it does a number on creeping bent and nimblewill. Mesotrione will be great for creeping bentgrass but it is not approved for residential lawns as yet so even a professional cannot apply it for you.

Thank you. I wasn't sure on the first one if it was poa or seeding fescue since it's only showing up in the fertilized areas from the dogs which that grass is growing so much faster than the rest. I tried looking at pictures but just couldn't pinpoint it to be poa or not. I'm planning on overseeding this fall so I'm not sure what to do about it and I have 25,000 sf so it seems like tenacity would be extremely expensive. Does that certain weed b gone also cover the other broadleaf weeds that the regular product controls or do I need to spray both?

I had some undesirable Tall Fescue patches mixed into my KBG lawn. I just used round up on those patches, which killed it off in about 3 days. You could do one round up treatment the first week and a second treatment a week later if you really want to. Round up should kill it all off in one treatment. Once it is dead, go ahead and rake out the dead grass or dig it out depending on how big the area is. If you rake it out, try to rake it just enough to where there is a little dead grass left and mostly soil. You could sprinkle a little top soil on the area before you seed it. Depending on where you live, you could probably do this whole process near the end of April to early May. Depending on the seed type you use, it’ll start to germinate between end of May to mid June. That should give it enough time to germinate and grow tall enough to start cutting before the summer time heat kicks in.