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Efficacy of Immunocontraception in Feral Horses
Largely unchecked by natural predators, many wild horse populations grow at rates of 18–25 percent per year. This unregulated growth can overtax the vegetation upon which their health and survival depend. To assist the Bureau of Land Management, the primary federal agency responsible for managing wild horses on public lands, in controlling herd growth and minimizing impacts to western ranges, USGS scientists have been testing the effects of two forms of the fertility control agent porcine zona pellucida (PZP) in three herds of feral horses. (The liquid form of PZP requires annual boosters, while the pellet form is designed to last for two years.) Study results were recently published as the cover story in the December 2011 issue of the Wildlife Society Bulletin. Controlling for age of mares and pretreatment differences in fertility, PZP significantly reduced foaling rates in all 3 herds, although the pellet form was less effective than pellets used in a previous trial and produced by an alternative method. Although PZP might be a useful tool in controlling fertility in some western U.S. horse herds, reduction in population growth rates will depend on timely access to mares for inoculation and the proportion of mares that can be successfully treated. The results of these studies can help managers weigh the implementation challenges of inoculating mares with PZP against the benefit of reducing population growth.
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