Wild Horse and Burro Population Management

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Wild horse populations often increase at high rates on U.S. western rangelands, which in turn can lead to habitat degradation. The U.S. Geological Survey and the Bureau of Land Management are cooperating on studies investigating the potential of fertility control drugs to reduce foaling rates. In addition, because nearly every management issue concerning wild horses depends on accurate herd counts, USGS and BLM are testing several techniques that could improve population estimates and provide defensible error estimates (confidence limits).

Image: Feral Horse (Equus caballus)

A feral horse standing in a field. John J. Mosesso, USGS. Public domain.

Molecular tagging is a new application of molecular genetic techniques to traditional mark-recapture methodology designed to address situations where traditional methods fail. In such studies, non-invasively collected samples (such as feces, feathers, or fur) are used as a source of DNA that is then genotyped at multiple loci such that each individual animal can be uniquely identified. Thus, each individual’s DNA represents a unique tag analogous to a band or other mark used in traditional mark-recapture studies.

There is a stated need for robust, repeatable, and transparent methods to estimate the size of free-roaming horse populations. Current population estimates are mostly based on aerial surveys, which are expensive, risky, and can be biased if protocols are not carefully followed or if a substantial proportion of the population is essentially invisible to observers. This study involves developing a new method for estimating horse population sizes based on non-invasive techniques; using genetic analysis of fecal samples (dung) and mark-recapture population estimation models. This research is in collaboration with Colorado State University and BLM.

FORT scientists are leading collaborative research projects to provide the BLM with better tools for managing expanding wild horse and burro populations. We are assessing the carrying capacity of wild horse habitats, behavioral effects of spaying mares and gelding a proportion of a herd’s stallions, testing the efficacy of an intrauterine device for mares, and evaluating four fertility-control tools for burros. We are also developing aerial survey techniques for burros, testing the use of DNA extracted from dung to count wild horses, and evaluating effects of cattle versus horses on sage- grouse habitat. Collaborators include Colorado State University, BLM, Oklahoma State University, University of Massachusetts, and APHIS.

A wild horse herd running in the snow.