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More than 280 bison were hazed back into Yellowstone National Park on Wednesday and Thursday, raising protests from a bison advocacy group about disrupting the animals during calving season.

Members of the Buffalo Field Campaign said Thursday that the hazing in recent days and weeks prompted at least one bison to give birth to a dead calf. The cumulative stress is putting unnecessary strain on the bison as they move out of the park to give birth, said Ted Fellman, a BFC spokesman.

"I can't imagine that getting ready to give birth and running seven miles, if that's conducive to a healthy birthing process," Fellman said.

BFC representatives said they saw a pregnant bison west of Yellowstone at Horse Butte that appeared to have a stillborn birth on Wednesday after hazing operations.

But Montana Department of Livestock officials said the calf was already dead on the ground, an apparent stillbirth, when they arrived for the hazing Wednesday morning.

"It was dead upon our arrival," said Karen Cooper, a DOL spokeswoman.

The hazing program is part of a state and federal effort to keep bison inside Yellowstone and reduce the risk of spreading brucellosis, a disease that can cause abortions, from spreading to cattle. There is no documented case of bison spreading the disease to cattle in the wild, but ranchers and others are worried about the risk of losing the state's brucellosis-free status.

The plan requires that wild bison that haven't been tested for brucellosis, or those who have tested positive, be kept out of Montana.

So far this year, 691 bison have been hazed back into the Yellowstone from the west side of the park, including 213 on Wednesday and 71 on Thursday, according to Cooper. Often, it's the same herd trying to leave the park.

About 230 bison were sent to slaughter earlier this year after leaving or nearly leaving the north end of Yellowstone.

Cattle are not roaming the Horse Butte allotment on public land and won't show up on nearby private land until June.

Cooper said the DOL and other agencies are required under the plan to keep bison inside of the park, whether cattle are present or not.

But BFC representatives questioned why bison are being driven away when there's no chance of coming in contact with cattle in the area. Repeatedly hazing pregnant bison may also be causing more stillborn births than has been documented, they said.

"There's some very pregnant bison out there," Fellman said. In some cases, "some of the calves' first steps are being hazed back into the park."

Cooper said that as of late Thursday afternoon, there were still about 10 bison wandering outside of the park on the west side.