Population dynamics and fertility control of feral horses

The management of free‐roaming, feral horse (Equus caballus) populations in the western United States is a contentious and challenging issue. Between 2008 and 2014, 1,873 individual horses from a closed population in the northern Great Basin, USA were captured, uniquely marked, and were either removed for private adoption or had permanent fertility control treatments (i.e., vasectomy or ovariectomy) applied prior to release. We derived the annual population size, growth rate, and the number of horses by sex and age by using the horse's estimated age at first capture to infer its age during each year it was part of the population. Estimates of population size from 2 aerial survey techniques averaged within 5.3%–9.6% of derived population estimates. The overall survival (i.e., recapture) rate for individual horses was 88% for individuals released back into the population between 2009 and 2012. Treated horses maintained group associations and there were no differences between survival rates for released females or males that received fertility control treatment, compared to animals released without treatment. The application of combined fertility control efforts resulted in a decline in the population‐level annual foaling rate from >20% to <4% within 4 years. Maintaining a proportion of a population as permanently non‐reproducing has the potential to safely maintain free‐roaming horse herds at desired management levels.