How to safely kill weeds in my duck pen??

I have some creeping charley (or whatever its called)
taking over the grassy area in a very large pen.
Short of digging every one up how can kill it without harming the
ducks in the pen?
Thanks for any advice.
CR

You could cover it with something that blocks light & water to smother it. A big old piece of vinyl flooring would do the trick, weighed down with rocks. (Cardboard or something that breaks down naturally might get messy with the ducks.)

Maybe vinegar?

Or you could corral the ducks to one side and take a propane torch to it. (Hit it with the flame until it wilts a bit.) You'd probably have to do it a few times, but that would work.

Thanks!