With spring in the air, and a lease deal hammered out on a quarantine facility, Yellowstone National Park began testing bison for brucellosis Wednesday, and plans to start holding disease-free animals at the Stephens Creek facility for a return to the park.

"What we've done up to this point is send every animal we captured to slaughter without testing," Al Nash, the park's public affairs officer, said. "We know that we can hold some bison for a period of time and then successfully release them in the park when things green up."

That time period is now, he said. So the management operation shifted Wednesday, with about 90 bison being held at Stephens Creek. Forty of the animals were calves that were tested, twice, for brucellosis, a disease that causes bison and cattle to abort.

By the end of the day, many of those calves had made their way to the quarantine facility near Corwin Springs, where federal and state researchers are trying to raise disease-free bison that might eventually be used to populate public lands in other areas.

"As we speak, I'm looking at a few of those bison up in the pasture," landowner Hunter Michelbrink said Wednesday evening. "I honestly couldn't tell you how many they brought in this afternoon."

Michelbrink last week signed a new three-year lease with the U.S. Department of Agriculture's Animal and Plant Health Inspection Service, so that his land, a former elk ranch, could still be used as a quarantine facility. The previous lease expired Jan. 31.

With that lease in place, the Park Service was able to start testing calves. Up to 100 calves that test sero-negative for brucellosis will be sent to the facility. The study is being jointly conducted by the federal inspection agency and the Montana Fish, Wildlife and Parks department.

"That agreement between APHIS and the landowner has been completed. That allowed us then to test calves," Nash said Wednesday. "This coincided with our analysis of the time frame that we could hold bison at Stephens Creek."

Bison testing positive for brucellosis will still be sent to slaughter. The Park Service has already sent 884 bison from Stephens Creek, with the meat distributed to food assistance programs.

But now, as long as there is room, bison testing free of the disease will be held for a return to the park about mid April.

Nash said the Stephens Creek facility will be able to hold between 200 and 400 bison for the next three to four weeks. By that time, plant growth in the park should be green enough to provide forage for the animals.

"We don't know how many bison may come into the Gardiner-Mammoth area in the coming weeks," Nash said. "We've yet to see any mass migrations of animals, but we've seen consistent movements of groups of 20 to 30."

The capture and slaughter procedure is part of the Interagency Bison Management Plan adopted in 2000, which is designed to conserve a viable, wild bison population while cooperating to protect Montana's brucellosis-free status. That means keeping bison separated from cattle present on land outside the park.

The plan has been the target of much criticism, particularly this year as the number of bison slaughter neared record numbers. The Park Service estimates that as of late summer 2007, there were about 4,700 bison in two herds in Yellowstone.