The AVMA endorses the American Association of Equine Practitioners’ (AAEP) position statement on "Tail Alteration in Horses," which reads as follows:
“The American Association of Equine Practitioners is opposed to the alteration of the tail of the horse for cosmetic or competitive purposes. This includes, but is not limited to, docking, nicking (i.e., cutting) and blocking. When performed for cosmetic purposes, these procedures do not contribute to the health of the horse and are primarily used for gain in the show ring (nicking/cutting, blocking and docking) or because of historical custom (docking). When a horse’s tail becomes injured or diseased and requires medical or surgical intervention for the health of the horse, it should be performed by a licensed veterinarian.
The AAEP encourages all breed associations and disciplines to establish and enforce guidelines to eliminate these practices. Members of AAEP should educate their clients about the potential health risks and welfare concerns involving these procedures.”
Welfare Implications of Horse Tail Modifications (PDF)