Breeder if you're not trading with loads of mules and stumpter horses. Dude up there said that you need horses for footmen. Indeed. However, theres a tolerance (which idk how much % it is), you can have extra horses without penalty. Most of them will be ridden by your footmen and you can also keep a safe % for future promoting higher tier cav and recruiting more troops. I haven't had the penalty for a while now and I always keep about 10-20% extra regular and war horses, but not stumpter or mule.
