Foaling rates in feral horses treated with the immunocontraceptive porcine zona pellucida

Authors

  • Jason I. Ransom,

    Corresponding author
    1. United States Geological Survey, Fort Collins Science Center, 2150 Centre Avenue, Building C, Fort Collins, CO 80526, USA
    • United States Geological Survey, Fort Collins Science Center, 2150 Centre Avenue, Building C, Fort Collins, CO 80526, USA.
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  • James E. Roelle,

    1. United States Geological Survey, Fort Collins Science Center, 2150 Centre Avenue, Building C, Fort Collins, CO 80526, USA
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  • Brian S. Cade,

    1. United States Geological Survey, Fort Collins Science Center, 2150 Centre Avenue, Building C, Fort Collins, CO 80526, USA
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  • Linda Coates-Markle,

    1. United States Bureau of Land Management, 10030 W 27th Avenue, Wheat Ridge, CO 80215, USA
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  • Albert J. Kane

    1. United States Department of Agriculture Animal and Plant Health Inspection Service, 2150 Centre Avenue, Building B, Fort Collins, CO 80526, USA
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  • Associate Editor: Messmer.

Abstract

Locally abundant feral horses (Equus caballus) can rapidly deplete available resources. Fertility control agents present promising nonlethal tools for reducing their population growth rates. We tested the effect of 2 forms of the immunocontraceptive porcine zona pellucida (PZP) on foaling rates in 3 populations of feral horses in the western United States. A liquid form requiring annual boosters was administered at Little Book Cliffs Wild Horse Range, Mesa County (CO), and Pryor Mountain Wild Horse Range, Bighorn County (WY) and Carbon County (MT), and a time-release pellet form designed to produce 2 yr of infertility was administered at McCullough Peaks Herd Management Area, Park County (WY). Average foaling rates (foals born/mare-yr) from direct observation of untreated and treated female horses (mares), 2004–2008, were 60.1% (n = 153 mare-yr) versus 6.6% (n = 91 mare-yr) at Little Book Cliffs, and 62.8% (n = 129 mare-yr) versus 17.7% (n = 79 mare-yr) at Pryor Mountain, respectively. At McCullough Peaks, mean annual foaling rates from 2006 to 2008 were 75.0% (n = 48 mare-yr) for untreated mares and 31.7% (n = 101 mare-yr) for treated mares. Controlling for age of mares and pretreatment differences in fertility, PZP reduced foaling rates in all 3 herds. The pellets used at McCullough Peaks (produced by cold evaporation) were less effective than pellets used in a previous trial and produced by heat extrusion. Immunocontraception with PZP may be a useful tool in reducing fertility rates in some western United States feral horse herds, but population growth reduction will depend on timely access to mares for inoculation and the proportion of mares that can be successfully treated. Published 2011. This article is a U.S. Government work and is in the public domain in the USA.

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