Feral horses are free-ranging descendants of once-domesticated horses. All free-ranging horses in North America are feral horses, and between 2014 and 2015, the feral horse population in the U.S. increased 18% according to the Bureau of Land Management (BLM). In 2015, the number of feral horses in the western ranges of the U.S. alone was estimated at 58,150. With few natural predators, populations will continue to rise, doubling every four years; thus, managing populations of feral horses represents a unique challenge in the U.S.
"The ever-expanding population of feral horses is a critical but not simple problem to solve," said Lori L. Ward, lead author of a recent review of the feral horse issue in The Professional Animal Scientist. "Any solution to this problem must have an understanding of current populations of horses in each ecosystem, the carrying capacity of the ecosystem and consideration of how these numbers will naturally vary."
In the 1950s, to combat rising populations, many feral horses were slaughtered by various means, including poisoned watering holes. This solution was met with public outrage and led to congressional action in the form of the Wild Free Roaming Horse & Burro Act of 1971, which protected feral horses as a link to our national heritage. This act protected horses on federal land and kept them from slaughter, prompting new efforts at population control.
Since that time, BLM has herded animals into holding locations where they can be managed and adopted-out. Many horses do not get adopted, however, and are labeled unwanted; likewise, the process of rounding-up feral horses is expensive and costs to maintain captive feral horses are estimated to exceed $1 billion by 2030. Such rising costs may end the adoption practice in coming years, according to the BLM.
Another means to limit populations is contraceptive use. The practice is controversial, as animal welfare activists often do not agree with the use of contraceptives, but The Humane Society of the United States is in support of such measures. Contraceptives, such as porcine zona pellucida vaccine, castration or vasectomy, have not been without side effects, however. These methods may only slow growth, or in the case of vasectomy have no effect on foal rates; likewise, they may disrupt seasonal patterns within the herds among other changes.
Perceptions of feral horses in the U.S. are numerous and multifaceted, which creates a unique challenge when it comes to managing their populations. In order to determine the most effective management practices, knowledge of horse population dynamics as well as public political views are necessary. Any solution to such an issue can only be gained by continued research.
The review article can be found at this link: http://www.professionalanimalscientist.org/article/S1080-7446(16)30080-8/abstract.