Mother cows with calves are the most dangerous animals to herd.

Evidence

The bulls were quite easily managed, but the cows with calves, old or young, were most troublesome. By accident I rode between one cow and her calf in a narrow place, and the cow rushed at me and was just getting her big horns under the horse, when he reared, and spun dexterously aside. This kind of thing happened continually. — Isabella L. Bird, A Lady’s Life in the Rocky Mountains, 150

Categorization

• STORYSTEM: A cattle drive scene where the docile bulls are fine but a maternal cow turns the trail into a near-death gauntlet — a natural set piece for a Western narrative centered on an outsider rider.
• XREF: Echoes Isak Dinesen’s account of how the tame and the wild blur in working animals; also connects to any ranching memoir where maternal instinct, not bull aggression, is the real hazard.