In Colorado, mustang numbers are kept in check by volunteers who prowl the range, shooting female horses with darts loaded with fertility-control drugs.

Advocates say that it’s working and spares Colorado wild horse herds from helicopter roundups that have ignited controversy in Wyoming, Nevada and other Western states.

The effectiveness of fertility-control darting is extremely limited in Wyoming, where the rangelands are vast and the mustang herds much larger than those in Colorado, a retired Bureau of Land Management (BLM) official said.

When mustangs are darted in Wyoming, it’s done by BLM personnel, not volunteers.

Volunteers used to dart wild horse mares (adult females) in Wyoming, but due to “concerns for safety,” that practice was stopped, BLM Wyoming spokeswoman Allegra Keenoo

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