I have a terrible ant problem also. I cant tell you how much money Ive spent trying to get rid of them. Ive tried DE and thats a joke, it didnt work for me. I have them in my house, my yard, trees, flower beds and even in my vehicle. I finally broke down and called an exterminator last week. He didnt treat the yard, just the house and outside parameter.
I found a poison made by Spectracide called Triazicide. Its a liquid that you hook to your garden hose to treat the yard and it seems to be working. I didnt let my chickens out for two weeks after I treated the yard just to be safe, and its rained a couple of times since then. I did buy bait traps to put in the coop and attached them high enough out of the chickens reach. So far its been working great. Hopefully my ant nightmare is over. Good luck!