Wild mustangs in the American West are descended from escaped domestic horses and face intense management controversy between ranchers, conservationists, and animal welfare advocates.
Wild horse roundups using helicopter pressure cause extreme stress and injuries including fractures, heart failure, and death. Horses are chased at speed and funneled into traps. Mares with foals are particularly vulnerable as foals cannot maintain the pace required. Holding facilities housing tens of thousands of horses in large open pens provide basic welfare but deny natural behavior and social structures. Fertility control using porcine zona pellucida vaccines is a welfare-positive alternative to roundups that is effective when consistently applied. PZP vaccination programs have reduced population growth significantly in test areas. The ongoing controversy between livestock interests wanting fewer horses and welfare advocates wanting retained wild herds requires evidence-based compromise that prioritizes individual horse welfare.