PHOENIX — Arizona should complete an independent scientific review before reducing a free-roaming horse herd below 150 animals, according to one of the nation’s leading equine population geneticists.
Dr. E. Gus Cothran, a renowned equine population geneticist and professor emeritus at Texas A&M University, said a population of 150 to 200 horses is generally considered the minimum needed to protect genetic health over the long term.
“At the most basic level, 150 to 200 animals is a minimum number,” Cothran said. “This is the number that, if everything goes right and only population genetics is involved, should maintain genetic health for 200 years or so.”
Cothran cautioned that there is no single number that guarantees a herd’s survival. The outcome also depends on the ages and sexes of the horses, how many mares and stallions reproduce, the herd’s existing genetic diversity and whether unrelated horses enter the population.
However, his answer provides an important scientific warning against reducing a herd far below the 150-to-200 range without first studying the consequences.
That means a small herd might continue producing foals for a period of time, but that does not guarantee it will remain genetically healthy over the long term.
Cothran said a population that remains too small will likely experience inbreeding and related losses in vigor, fertility and viability, although it is impossible to predict exactly how quickly those problems would appear.
In plain language, a herd that becomes too small may eventually produce fewer healthy foals, become less able to adapt to disease or environmental changes and require increasing human intervention to survive.
“Genetic testing can give a good estimate of genetic history, which likely can be used to determine if the herd has a history of isolation in the area,” Cothran said.
He said testing 20% to 25% of a herd would provide a reasonable sample.
The findings do not establish a specific carrying capacity or management number for any individual Arizona herd. That would require herd-specific genetic testing, population records, reproductive data and a scientific evaluation of the habitat.
Supporters of the herd say the findings show why Arizona should not permanently remove horses based on an unsupported target, informal vote or administrative preference.
Friends of the Salt River Wild Horses is calling on Arizona officials to pause permanent removals until the state:
- Completes an independent genetic study of the herd;
- Releases the data and scientific method used to establish its proposed herd-size target;
- Determines whether that target will preserve a healthy, self-sustaining population; and
- Allows the findings to be reviewed by independent experts.
A herd can continue to exist for years while quietly losing the genetic diversity needed for its long-term survival. Once that diversity is lost, it is gone forever, claim supporters.
Supporters of the herd say that before a single horse is permanently removed, Arizona should demonstrate through independent science that the remaining herd will be genetically healthy and capable of surviving for generations. Friends of the Salt River Wild Horses calls upon Governor Hobbs to halt removals, to do the science, and respect the public process.
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