Fact Sheet on BLM's Wild Horse and Burro Fertility Control Program

The BLM has promoted and supported the development of an effective contraceptive agent for wild horses since 1978. The most promising agent is a Porcine Zona Pellucida (PZP) vaccine that was developed in the 1990s but is not commercially available.

PZP is used by the BLM under an investigational exemption issued by the Food and Drug Administration (FDA) and held by the Humane Society of the United States (HSUS).

The most effective formulation is a one-year vaccine that must be administered annually. However, it is not feasible to gather wild horse herds every year to administer the vaccine, and it is very difficult to approach most wild horses and burros on Western rangelands closely enough to allow darting. Instead, the BLM has been using a two-year pellet that must be administered to mares after they have been captured. In addition, maximum effectiveness is not achieved unless mares are treated during a three-to-four month window prior to foaling.

This also means more mares need to be captured and released than would normally be gathered and simply removed from the affected Herd Management Area.

Since 2004, the BLM has administered this pelleted vaccine to a total of 1,808 mares on 47 of its 199 Herd Management Areas, but significant reductions in population increase have not been apparent (see the table below).

The BLM believes there is potential for savings in reduced removal and holding costs through the use of fertility control on wild horses. When a herd is treated, the savings will not be realized or apparent until the next time the herd is gathered, which would normally be three to four years later.

In areas with large horse populations that are three to four times the appropriate management level (AML), it is very difficult to capture enough additional mares to treat and release back. Once enough horses have been captured to bring the population down to AML, catching the remaining horses becomes very difficult because they are scattered over a large area and some have learned to avoid the helicopter used in gathering.

Under ideal circumstances, and with the treatment of many mares in the herd, this agent may be able to lengthen the gather cycle by one year.

HSUS is currently working with the BLM to improve the drug and the means of applying it.

The BLM is continuing research to determine the population-control effects on the herds that have been treated and is looking into the possibility of a longer-acting agent. The BLM will also will continue to treat herds where practical; however, cost savings from reduced reproduction rates will not be realized in the immediate future.

FY			# of Mares Treated
2004	15 HMAs		763
2005	10 HMAs		423
2006	12 HMAs		333
2007	7 HMAs		133
	44 HMAs	2004-2007 Totals	1,654
2008	White Mountain HMA	WY	44
	Little Colorado HMA	WY	18
	New Pass/Ravenswood	NV	92
		2008 Totals	154
	47 HMAs	2004-2008 Totals	1,808