More than 100 Bureau of Land Management (BLM) mustang mares residing at the Northern Nevada Correctional Center’s holding facility in Carson City will be treated with a longer-lasting version of the immuno-contraceptive agent porcine zona pellucida (PZP) as part of a research study to determine if the drug can extend the animals’ infertility to up to four years.

More than 100 mustang mares, like these seen here, will take part in a study evaluating if PZP can render a mare infertile for up to four years.

The PZP vaccine currently used by the BLM is injected as a liquid or fed in pellet form to fertile mares aged 4 to 20 years. The contraceptive acts as a foreign protein against which treated mares produce antibodies that attach to the mare’s sperm receptors on the ovum and block fertilization. A single PZP vaccination currently renders treated mares infertile for 22 months