What does Warwick Schiller say about horses that nip or bite and what is the correct response?

Warwick Schiller addresses nipping and biting as behaviors that exist on a spectrum from low-level mouthy behavior in young horses to established biting in adult horses, and his approach to each differs based on the horse's history, the severity of the behavior, and what is driving it. For young horses that mouth, nip, or explore with their lips — behavior that is normal for foals and weanlings — Schiller teaches that the response needs to be immediate but proportionate. A sharp, quick movement away from the muzzle — a bump of the hand, a stern verbal response, a step into the horse's space — that interrupts the behavior without escalating into punishment. His concern with excessive correction of mouthy young horses is that heavy-handed responses can create head-shy horses or horses that become more agitated around the handler's hands. For adult horses with an established biting habit, Schiller distinguishes between a horse that bites from dominance — which bites deliberately and with clear intent — and a horse that bites from pain or anxiety — which bites when girthed, when touched in sensitive areas, or when it is stressed. The dominant biter needs a clear, consistent correction delivered at the moment of the attempt, not after the fact. The pain or anxiety biter needs the pain or anxiety source identified and addressed first; correcting the bite without addressing the cause typically produces a horse that bites with less warning rather than one that stops biting. Clinton Anderson's position on biting is direct: it must be corrected immediately and consistently every single time, because biting that is sometimes corrected and sometimes ignored teaches the horse that biting is worth trying. His correction is a quick, firm bump into the horse's space — not a hitting response, but a clear invasion of the horse's space that communicates the bite attempt was wrong. The correction must happen within one second of the attempt or it loses its connection to the behavior.