Wild bison allowed to migrate north of Yellowstone National Park for the first time in decades have strayed off their new grazing grounds three times since their arrival this week, park officials said Friday.
The 25 animals will be hazed back onto the Gallatin National Forest if they leave again, officials said, but it was uncertain what would happen if they continued to enter areas where they could encounter cattle.
Wildlife officials fear cows could be infected with a disease carried by the Yellowstone bison. Brucellosis can cause animals to prematurely abort their young.
For years, bison trying to leave Yellowstone to graze at lower elevations in Montana were captured and slaughtered to prevent the spread of brucellosis. The 25 bison on the Gallatin — part of a pilot program that was years in the making — tested negative for the disease.
Those animals are part of an initiative to expand areas outside the park where bison can roam.
Hundreds of bison attempt to leave the park during harsh winters. With snow piling deep this year, 63 of them that tried to leave the park in recent days have been captured at the park's northern boundary.
The animals are being held in a Park Service facility near Gardiner. They could either be eventually returned to the park or sent to slaughter.
At least 19 tested positive for exposure to brucellosis, including some that had been considered for movement onto the Gallatin Forest. Park officials earlier pledged those animals would be let free, but said that might not be the case anymore.
"I may have misspoke, and I'll own up to that," Yellowstone spokesman Al Nash said. "We have not made a decision on any bison we're holding."
Nash added that a meeting was planned among park bison managers next week to discuss what will happen if the 25 bison continue to leave their 2,500-acre grazing grounds within the Gallatin Forest.
Wildlife officials had hoped the animals would be contained by natural features including the Yellowstone River. But about a dozen animals have crossed the river three times since Wednesday.
John Youngberg with the Montana Farm Bureau said it could prove impossible to keep them contained. He pointed out that the latency period for brucellosis is unknown, although there has never been a recorded bison-to-cattle transmission in the wild.
"How do you control them?" Youngberg asked. "They are a large, large ungulate. Fences don't seem to make a whole lot of difference with them."
Copyright 2011 The Associated Press.