The Bureau of Land Management (BLM) has proposed the use of a non-hormonal contraceptive, porcine zona pellucida (PZP), over the next five years to limit equine overpopulation of Pryor Mountain Wild Horse Range. BLM authorities say the range has been taxed by recent drought conditions.
 
In an Associated Press (AP) article, Linda Coates-Markle, BLM’s wild horse and burro specialist, said the BLM plans to administer the contraceptive to all mares 11 years of age and older (24 in 2006) until the year 2010 along with removing up to 24 of the more than 160 wild horses on the land (which is located in southern Montana and northern Wyoming) to lessen the herd’s impact on the pasture until a long-term solution can be made. The program could be suspended for one year if the herds have lower-than-average increases in population.

“We have to limit grazing impacts, at least temporarily, until we get out of the drought, and the range has a chance to show an improving trend,” she said in the article.
 
According to the USDA, PZP is an intramuscular immunocontraceptive agent that uses the immune system rather than hormones to prevent egg fertilization. It is derived from pig zona pellucida, a strong membrane that forms around an ovum as it develops in the ovary.

When injected, PZP causes the mare’s immune system to make antibodies against it; these then bind to the mare’s zona pellucida and prevent sperm from penetrating the membrane. Treated mares continue to cycle and ovulate normally, but are unable to conceive. The drug will not affect mares already in foal. A study conducted in a BLM herd in 2000 showed that PZP was 90% effective in preventing pregnancies in horses. Comments on the project will be accepted until May 5. (Comments can be submitted in writing accompanied by a signature to: Bureau of Land Management, Billings Field Office, 5001 Southgate Drive, Billings, MT, 59101.)

Dave Pauli, a regional director for the Humane Society of the United States that partnered with the BLM to administer the contraceptive, told the AP, “Long-term, we would rather have a safe, healthy herd than an overpopulated, stressed herd