BILLINGS -- Officials at Yellowstone National Park plan to hold about 300 bison captured Wednesday until there's sufficient greening up of grasses in the park to help keep the animals from wandering into Montana to look for forage, a park spokesman said.

Al Nash said it could be two to three weeks before the bison are released from the Stephens Creek capture site, just inside Yellowstone's northern edge.

This is the third time this winter that the park has opened the facility for bison captured near the border. The hazing and capture of bison is allowed under a state-federal management plan aimed at reducing the risk of migrating bison spreading the disease brucellosis to cattle in Montana.

Most of the bison that were captured near the northern boundary earlier this year were sent to slaughter without first being tested for brucellosis. Authorities have said this is an option if the park's bison population tops 3,000. The plan considers 3,000 the target population.

The late winter-early spring estimate released by the park earlier this month was 3,500 animals. The park said the estimate took into account such factors as management activities and winter mortality rates. Heading into winter, the estimate was 4,900 bison, a documented high.

Nash said holding the bison temporarily was the ''best option available to us right now,'' one that keeps bison from potentially mingling with cattle and allows for their eventual release. He said the captured bison would be in pastures.

''At this point, because of the time of year and because green-up is imminent, barring a very unusual winter storm, we expect we will be able to hold them for a very short period, knowing in the past we have been able to successfully release them back into the park in mid-April,'' Nash said.

Stephany Seay, a spokeswoman for the activist Buffalo Field Campaign, said her group was ''thrilled'' that the bison weren't going to slaughter. ''But it puts a lot of holes in their argument that brucellosis is the issue here,'' she said, noting that officials planned to release the bison without testing them for the disease. She believes cattle are given priority over bison.

Meanwhile, seven bison captured near Yellowstone's western border were sent to slaughter Wednesday without first being tested, state livestock department officials said. State veterinarian Tom Linfield said bison taken to slaughter houses this winter were being tested at those facilities.