Effectiveness of SpayVac-pZP or -GnRH in preventing pregnancy in bison cows

Female bison will generally reach sexual maturity and breed when they are two years of age.  Grass finishing of bison females requires maintaining individuals on range until they are 30-42 months of age; well past sexual maturity.  The Institute typically keeps a portion of female bison that are destined for grass markets in separate herds away from breeding bulls to prevent unwanted pregnancies in market animals.  However, managing multiple, separate herds can complicate grazing planning and potentially impact range conditions.  Thus, a treatment that prevents pregnancies in females exposed to bulls could be a useful management tool for bison producers.  Because many bison are marketed as hormone–free meat, hormone-based contraceptive methods, like synthetic progestins, are not an option.  Surgical treatments, like removing the ovaries, may be socially unacceptable and can be overly stressful on bison and lead to hemorrhaging and infection. Porcine zona pellucida (pZP)– and gonadotropin-releasing hormone (GnRH)-based contraceptive vaccines have shown promise in preventing pregnancies in overabundant wildlife populations such as white-tail deer and wild horses, as well as other species, including bison. However, booster doses are usually required. Unlike current vaccines on the market, SpayVac provides single-dose, long-lasting contraceptive for a variety of species which may be effective for 2-5 years in bison; likely long enough to prevent pregnancies in female market bison. This Institute project will compare the effectiveness of both pZP– and GnRH-based SpayVac vaccines to a control group in preventing pregnancies in young female bison for at least two years and document any potential side effects.