I have to wonder how it's designed - they don't want to make it too easy, I understand, and maybe the idea was that players would have trouble getting past certain points, but would keep trying (maybe by going elsewhere in the game for a while, which I'm doing right now) and eventually get through. Maybe there are only certain animals in this particular herd that can be broken?
I'll keep at it, as you suggest, but I'll try not to let it get to me. Finally succeeded at the Assault on Ft. Mercer mission, which I had abandoned for a while. Tough one to re-do, since I didn't have access to medicine, which seems to be the one thing I could have used, besides patience. Just have the cart race left (besides the remaining final missions), and I think I ought to get through it okay.