What is cowboy slang for bull mating?

Bulling. Sometimes there's a cow in heat that all the bulls want (applies only to herds where producers run more than one bull in), and mistakes can be made if the mating gets a bit intense between that one cow and multiple bulls who are all too intent on getting her bred. It is also found to be done in steers, usually when they are crowded, but also if there is a weaker animal in the herd that the more dominant steers feel the need to pick on all the time.