Female horses at the San Felipe Pueblo in New Mexico have started receiving a contraceptive vaccine to safely and humanely control the historic herd’s population.

The vaccine—porcine zona pellucida, or PZP—has been used successfully in other U.S. horse herds and has been used for decades in zoo animals.

“Safely managing the population of the herd is vital to keeping it healthy,” said Ricardo Ortiz, San Felipe Pueblo land management specialist. “PZP is safe and easy to administer and we’re confident, based on its success in other herds that it will help the San Felipe horses to thrive.”

The San Felipe Pueblo will employ PZP in accordance with the recommendations of a recent report by the National Academy of Sciences, and in accordance with the nonprofit Science and Conservation Center in Billings, Montana, which produces the vaccine