Using fertility control measures on the BLM-managed wild horses is an effective means of controlling the prolific animals' population, but that will not eliminate roundups, said one Bureau of Land Management official.The bureau is responsible for the wild-horse management program, and employees have their hands full caring for 33,000 wild horses and burros.

The MuCullough Peaks' 185 horses on 110,000 acres may not provide an ideal illustration, but it is a local herd under scrutiny.

In 2004, about 500 MuCullough Peaks horses were gathered by the bureau. Of that, around 100 males and females were returned to their range, said Alan Shepherd, BLM Wyoming wild horse specialist in Cheyenne.

It was about a 50-50 male-female ratio, he said.

Of the 50 or so mares, 36 were vaccinated in the corral with PZB — porcine zona pellucida, Shepherd said.

PZB allows a mare to ovulate, but the drug reacts with her immune system to prevent spermatozoon from entering the egg, Shepherd said.

PZB will last one to three years and is 80 percent effective or better, Shepherd said.

When the mares are confined in corrals, it is relatively easy to inoculate them with PZB. However, mustangs are canny critters and dodge capture, so the bureau usually catches about 85 percent of the herd during gathers, Shepherd said.

The normal wild-horse population growth rate is 20 percent per year. PZB slows growth to 10 to 12 percent per year, Shepherd said.

It costs $200 per mare to vaccinate with PZB. To round up, feed and transport a McCullough Peaks mustang, the price is around $600, Shepherd said.

Marshall Dominick, president of Friends Of A Legacy, a group dedicating itself to McCullough Peaks wild horses, is not totally sold on PZB. But he said if the drug proves safe, it could help avoid intrusions on the mustang herds of the West, particularly roundups. It could also save the bureau money by managing swelling horse numbers, he said.

“I think we need to proceed with caution,” Dominick said, “that is the key.”

PZB also reduces the number of gathers that the bureau undertakes to maintain the range.

The math is straight forward. Fewer spring foals means it takes longer to exceed appropriate management levels, which in turn reduces the number of roundups.

If McCullough Peaks mustangs are gathered this year or early next, Shepherd said tentatively, 30 to 35 mares would receive the PZB shot. There are approximately 185 wild horses. Of that 85 would be kept, and the remaining 100 mustangs returned to their range.

Dominick said in 2004, 34 mares were inoculated, and after four years, 16 mares have yet to foal.

“That is a red flag,” Dominick said.

Still, PZB has proven its worth where it was introduced on Assateague Island, Maryland. More than 15 years ago, equines that Dominick said are classified as ponies, were inoculated with PZB. Since then, the population has remained stable, and the National Park Service, which cares for those horses, has not removed any young animals.

Dominick said the Assateague ponies receive PZB field darts. Field darts are good for one year, and are more difficult to administer, Shepherd said.

The Assateague ponies' habitat differs from McCullough land, and Dominick said there may be diversity in the genes of the two distinct herds, so McCullough horses' reaction to the drug may be different than their cousins back east.

The U.S. Geological Survey is studying the McCullough herd now. Shepherd said the bureau will determine how many horses should be vaccinated.

The bureau has been using PZB since 1991, vaccinating horses from 50 different herds, Shepherd said.

Shepherd said experts continue to research the drug in an effort to improve its effectiveness and longevity.

If a roundup occurs, captured horses will be trucked to the bureau's horse holding facility in Rock Springs. This spring, the horses would be returned for a Big Horn Basin adoption.

“We still have horses to put into good homes,” Shepherd said.

Dominick, who owns 14 mustangs, agrees with Shepherd concerning adoption.