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Driving down a lonely Nevada highway, I saw their silhouettes, a small herd of horses on the crest of a hill. They were the wild horses I had hoped to see, living symbols of the old American West.

More than 40,000 wild horses and burros live on public rangelands in 10 western states. These herds have been protected since 1971 when the government passed legislation to manage them. At that time, wild horses on open ranges were fast disappearing; often considered pests, they were frequently slaughtered and sold to the horsemeat industry.

Today, the protected wild horse population is growing rapidly, which has caused new concerns. Herd sizes, if left unchecked, can double every four years. There is now concern that overabundant free-range wild horses are altering natural plant communities and reducing forage for native wildlife and livestock. Land management agencies periodically round up wild horses to adopt out or sell. Many horses are housed in long-term holding pastures. 

Fertility control has been used with moderate success to manage some wild horse herds, but the current method is effective only a short time and can cause undesirable side effects. Morris Animal Foundation–funded researchers at Colorado State University recently completed a three-year research project to evaluate an alternative method of fertility control in wild mares, a gonadotropin-releasing hormone contraceptive vaccine.

The vaccine proved safe, caused few side effects, and was about 50 percent effective in reducing foaling rates over a two-year period. This was an improvement over the previous method, but still not ideal. In addition, three years after injection, 95 percent of the mares had regained normal fertility, meaning repeat vaccinations would be necessary.

The Colorado State University team is now giving booster shots to the previously vaccinated mares to measure the long-term effects of re-immunization. The 2015 foaling season data will gauge the success of these current research efforts.

Morris Animal Foundation continues to invest in new strategies to improve the health and welfare of wild horses. Make a donation today and join our efforts to advance the health and welfare of all horses, domesticated and wild.