Are they heifers or steers? The heifers do not get as mean as the steers and the bulls are not workable.
We have kept bison for years. I have a heifer (I guess cow now) that I have had for 8 years. She has slowed down, but still turns any time a horse steps up in front of her shoulder and goes forward any time a horse is behind her shoulder. I also have a steer I have had for 5 years. He threatens a horse once in a while but does not charge. I put a steer in the freezer last fall that would charge some horses. Talk about smart -- he could read a horse in about a minute or two and would 'honor' it or charge it according to how 'bold' the horse was.
We have had bison for so long that we understand them pretty well. They are MUCH smarter than a horse or a cow either one. Husband is American Indian and he laughs at people saying 'there is a reason there were over 500 Indian Tribes when the Whites came and not one Indian was ever seen riding a buffalo.'
The reason they do not herd well is that their eyes are too close together to see anything behind them. So --- they turn and face anything that moves behind them. Pretty hard to drive a animal that does that. They are real suckers for food. Every set of bison we have had learned very quickly to come to a bucket of feed. A lot of bison will turn toward a horse when you work them -- every time. They do not charge; They just want to turn where they can see you.
There are many little worthless bits of information about bison that are interesting. Like:
They are 'true leaded' like a horse. They are so athletic that they seldom crossfire or 'cow-canter'. They can match any horse for athletic ability.
They can change leads as effortlessly as the best reiner and always change complete.
They have both lungs in one plural sack. Therefore, an Indian could get one arrow into a 2000# bull and if it went between ribs and punctured the plural cavity, both lungs would collapse and the bison bull would die within minutes.
They do not have cartilage rings around their wind-pipes, so if you rope them, about half of them will die even if you loosen up the rope. If we ever have to doctor one, we always catch one by roping both hind feet. We do not have a chute we can work them through.
Since their shaggy hair is on their front ends, they always face a blizzard or a cold wind.
I am sure there are a lot of other tidbits I have picked up over the years but I cannot remember them now. I know I have some pictures of horses working them. I think a couple of photos are in our website. I'll see if I can find them.