BILLINGS, Mont. -- A new study says more bison could be allowed to migrate outside Yellowstone National Park without significantly increasing the risk of spreading a disease carried by the animals to livestock.

Yellowstone's estimated 3,000 bison are periodically culled during their winter migration to prevent the spread of the disease brucellosis to cattle.

But in a study scheduled to appear in the February issue of the Journal of Applied Ecology, researchers argue much of that culling is unnecessary because there are few cattle in areas next to the park.

"If you could work out something with those ranches, you could let a lot more of those bison roam without increasing the risk," said lead author Marm Kilpatrick, an assistant professor at the University of California, Santa Cruz.

No transmissions of brucellosis from wild bison to cattle have been recorded. In the last several years, however, elk have been suspected of transmitting brucellosis to cattle at least seven times in the Yellowstone region of Montana, Idaho and Wyoming.

The disease causes pregnant animals to prematurely abort their young and lose weight, which can depress a cow's market value. It is transmitted through contact with birthing materials.

The new study was based on disease transmission probabilities and bison movement patterns.

Kilpatrick and two colleagues ran computer models that indicated the bison population growth over time leads to a higher probability of disease transmission.

But because the risk remains relatively low even as bison numbers rise, Kilpatrick concluded the easiest way to solve the problem was to reduce opportunities for the animals to interact with cattle.

He said buying grazing rights from ranches near the park could be the most cost effective solution.

That idea was put into practice in December with the leasing of grazing rights on the Royal Teton Ranch. That 30-year deal, brokered by Montana officials, will give $3.3 million to the ranch's owners in exchange for a fenced bison corridor through the property.

Up to 100 bison could use the corridor in future years to access public land outside the park.

"It's easy if you're a cattle person to see bison and say, my livelihood is under threat here," Kilpatrick said. "But in the meantime, what's the chance of that cow coming into contact with an infected birth site? That's a really, really low number."

Fewer than 1,000 cattle graze in areas where Yellowstone's bison typically migrate, Kilpatrick said. That number drops to fewer than 300 head in the winter, when some areas west of the park have no cattle at all.

A veterinarian with the U.S. Department of Agriculture, which oversees livestock disease prevention, said Kilpatrick's computer modeling results appeared to be accurate. However, veterinarian Jack Rhyan said those results do not necessarily point to the purchase of grazing rights as the best option.

"That will buy us five years while (bison) expand into that range, until they reach out farther," Rhyan said. "It's a temporary way to take the heat off of the problem ... but that just allows the problem to get that much bigger."

Rhyan said the long-term solution still rests in getting rid of the disease in wildlife.

State and federal agencies including Rhyan's have shown more tolerance toward bison in recent winters when cattle are absent. However, they've also continued to enforce a capture and slaughter program that calls for killing bison that roam beyond set boundaries.

Conservation groups have tried to expand those boundaries, while livestock interests have pushed for them to be more strictly enforced. As long as brucellosis exists in bison, some ranchers say, cattle remain at risk.

Kilpatrick's study was funded by the Wilburforce Foundation and the National Fish and Wildlife Foundation. It was co-authored by Colin Gillin of the Oregon Department of Fish and Wildlife and Peter Daszak at the Consortium for Conservation Medicine.