Can any western horse compete in working cow horse?

Technically any western horse can be entered in working cow horse competition, but the practical requirements of the discipline mean that horses without specific natural and trained qualities will struggle significantly in the cattle work phases regardless of how well they perform the reining portion of the run. A horse that lacks natural cow sense — the instinctive interest in tracking and controlling cattle — can learn to participate in cattle work through training, but it will rarely produce the athletic, instinct-driven fence work that judges reward with above-average scores, because the horse is responding to the rider's placement rather than moving from its own desire to control the cow. A horse that lacks the athletic ability to slide to a correct stop, make a correct turn on the fence at speed, or rate its pace precisely behind a cow will be limited in the quality of cattle work it can produce regardless of its training quality or cow interest. Quarter horses dominate working cow horse competition because the breeding selection that has produced the modern working quarter horse specifically targets the combination of cow sense, stop, and athletic ability that the discipline demands, but horses of other breeds with exceptional natural cow sense and athletic ability have competed successfully. The practical answer is that a horse needs both genuine cow sense and the athletic foundation to work cattle correctly in order to compete at any level beyond the most introductory classes, and those qualities together are not common across the general western horse population.