They are quite dangerous when with their calves. Try not to get inbetween mother and calf. Otherwise they are o.k.
I think all the stories we hear about cows killing people is the fact that a dog was around. So on an mtb you should be o.k (mainly no dog).
There is that fantastic video of a rider at Megavalanche riding in a cow suit.
As a child we lived opposite a farm and used to ride the cows, looking back this probably wasn’t a good idea!