Fertility Control

The BLM has supported the development of an effective contraceptive agent for wild horses since 1978. Over the years, several approaches -- such as hormone implants, chemical vasectomies, and intrauterine devices -- were tried, but abandoned as ineffective or impractical. Currently the most promising agents are porcine zona pellucida (PZP) vaccines that were developed starting in the 1990s. The BLM currently uses PZP in two formulations. The most effective is a one-year liquid vaccine that must be re-administered annually. This vaccine, known as ZonaStat-H, was registered in February 2012 with the Environmental Protection Agency for preventing pregnancy in wild horse mares. However, it is not feasible to gather wild horse herds every year to administer this form of the vaccine. In a few herds, horses can be treated by darting each year, but darting is generally not practical for BLM because it is difficult to approach most wild horses closely enough on Western rangelands. For this reason, the BLM mostly uses a longer-lasting, 22-month, pelleted PZP agent (PZP-22). The pelleted vaccine has been successfully administered by darting into a few mares, but typically it is hand-injected after the mares have been captured. This method of treatment means that during gathers, more mares need to be captured (for treatment and release) than would actually be removed from the range if removal was the only goal. While this is usually possible, it can be difficult to capture a large enough fraction of the population so that significant numbers can be treated and released. Maximum effectiveness of PZP-22 is achieved when the mares are treated during a three- to four-month window prior to foaling.

Since 2004, the BLM has administered the pelleted PZP vaccine to more than 4,562 mares on 80 of its 179 herd management areas (HMAs), but significant reductions in the rate of population increase have not yet been apparent. Analysis of data from the McCullough Peaks herd, which was treated in 2004, indicated that treated mares had an average foaling rate of 32 percent in the two years following treatment, compared with a 75 percent foaling rate in untreated mares. 

The BLM believes that there is potential for savings in reduced removal and holding costs through the use of fertility control in wild horses. How great these savings will be when the treatments are widely applied in the field remains to be seen. When a herd is treated, these savings will not be realized or apparent until the next time the herd is gathered, which would normally be three to five years later. 

The Challenge of Large Populations

In areas with large horse populations that are three to four times the appropriate management level (AML), it is difficult to capture enough additional mares to treat a significant number and release them back to the range. Once enough horses have been captured to bring the population down to AML, catching the small number of remaining horses becomes challenging because they are scattered over larger areas and many have become more evasive. More mares could be treated if herds at AML were gathered for the primary purpose of fertility-control treatment with no or limited removal. This approach, called a catch-treat-release (CTR) gather, is currently being implemented by the BLM on HMAs that are at or near AML.

In 2008, the BLM approved a research proposal from the Humane Society of the United States to study the efficacy of the 22-month vaccine in a collaborative project with BLM. The study aims to investigate new ways to apply the vaccine, as well as measure the efficacy and behavioral effects on treated mares. Two HMAS are enrolled in the study -- the Cedar Mountain herd in Utah and the Sand Wash herd in Colorado. This work is supported under a research grant from the Annenberg Foundation. Preliminary results suggest that in some HMAs most of the mares can be darted with the PZP-22 treatment, but in others darting is not possible. Early results also suggest that the treatments reduced foaling by about one-third to two-thirds in these two populations. With these foaling rates, population growth was slowed but still significant over time.  

The BLM is continuing research to determine the effects of fertility control on the rates of population growth and to develop longer-acting agents. The BLM will also continue to treat herds where practical, but cost savings from reduced reproduction rates will not be realized in the immediate future.


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