The pasture breeding study will test the effectiveness of two formulations of the investigational contraceptive vaccine, SpayVac, to determine if the treatment can reduce foaling rates in wild horse mares.
The goal is to see if SpayVac, a novel formulation of a glycoprotein called porcine zona pellucida (PZP), will provide a longer-term effect than other PZP vaccines currently used by the BLM.
If the vaccine is found to reduce foaling in this controlled setting, it will be considered for use with free-roaming horses to help control population growth rates on the range.
The bureau has signalled its plan to use long-term contraceptives more aggressively to control wild horse numbers. Horses over and above appropriate management levels have been gathered from the western rangelands, and now number close to 40,000 in captivity.
There are now more in captivity than live wild in the rangelands, according to estimates.
The bureau acknowledges it has a need for a long-lasting contraceptive agent to control herd growth rates, saying wild horse populations increase at an average rate of 20 per cent a year and can quickly exceed the carrying capacity of their ranges.
A main limitation of the long-term contraceptives currently available is that they are of relatively short duration or need to be administered annually.
Maximising the duration of contraceptive effectiveness is especially important in wild horses, which in most cases must be captured in order for the treatment to be successfully administered.
In the study, 90 mares have been treated with either one of two formulations of the vaccine or a placebo.
The mares will be followed for five years to measure anti-PZP antibody levels and compare the foaling rates between treated horses and controls.
Although breeding is not usually allowed to occur in BLM facilities, a clinical trial in this controlled environment will provide critical information on how well SpayVac works as a contraceptive.
The mares and stallions enrolled in the study were selected from horses already in bureau holding facilities.
They are being housed in three 30-acre pastures and will be together during the next five breeding seasons.
Foals that are born during the study will be offered for adoption each fall after they have been weaned. At the conclusion of the study, all adult horses will be returned to the BLM's Adopt-A-Horse Programme or placed in long-term pasture facilities.
The study is a collaborative effort with scientists from the United States Geologicial Survey, veterinarians with the Department of Agriculture's Animal and Plant Health Inspection Service, and TerraMar Environmental Research.