Most years, when I've seen it being a problem, we've been in a prolonged drought situation, and it's one of the few 'green' things still available... usually growing right in the edge of the woods, or shady, low-lying spots.
I never lost any of my own to it, but didn't have a lot of it, and a lot of times, if we were getting into the sort of pasture conditions that favored them even thinking about eating it, I'd go in ahead of the cows and clip/weedeat any present in the paddock they were about to move to.