Cows (and bulls) only charge if provoked. If there's no reason to charge, then they're not going to waste their energy doing so. It's that simple.
There is no actual statistic or number that depicts how often cow will charge. It's kind of a silly thing because it depends on the individual cow in terms of her personality, as well as the situation she's put in. A cow is more likely to charge if she's in a situation where she has a baby calf to protect and such a situation is one where she's not pampered in a nice barn somewhere. She's more likely to charge because of that calf than if she didn't have a calf with her; she's much more likely to charge if she has the personality of an over-protective momma than if she had the personality where she cared about her calf, but doesn't mind having her human caretakers handling it.
A cow will charge if she feels cornered with no escape route, and she's deliberately put in a situation where she feels that doing so is necessary for her own survival. How often this happens again, depends on her personality, on the situation, not to mention how accustomed she actually is to humans or horses or dogs. The same can be said for bulls.
Normally, most of the time (possibly 98% of her life) a cow is much more liable to live in a much more calm state where their greatest amount of energy spent is walking from water to pasture and back again, grazing, and ruminating. Very little other time is spent coming after, or even running away, from certain potential perceived threats and dangers to her and/or her herd mates.
The Bovine Practicum Q&A "blog" is an informative, just-for-fun section where I find a variety of questions that are often (and not-so-often) asked by inquisitive people like you, and answer them to the best of my ability. Much of the questions have come from a wide variety of reaches from the Internet.
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