Three weeks since a May 15 deadline to haze wild bison out of Montana and into Yellowstone National Park has passed, government agents are still running operations to clear bison out of the state.

Officials point to the large number of bison that left the park over the winter to explain hazing continuing with less than a week before cattle begin to be grazed on ranches in the area. But others say it is a sign that the park is not yet lush enough to sustain bison and that the animals should have been left alone until later in the year.

"We've had bison come out repeatedly since May 15," said state veterinarian Marty Zaluski with the Department of Livestock. "One of the things that we're coming to recognize is what a logistical challenge it is to return 600 to 700 bison back into the park in an organized and speedy, responsible manner."

Starting last year, state and federal agencies have allowed more bison to cross Yellowstone's western border and graze on pasture around Hebgen Lake. The policy won praise from environmentalists and irked ranchers, but Zaluski declined to say what or whether officials will do anything different next year given the current challenges.

"I think it's a bit premature to make conclusions how that impacts next year's operations," he said, "particularly since we haven't really finished. We hope we're done, but we're not necessarily finished."

Government agents pushed 40 bison into the park on Monday, Zaluski said.

As government workers ramped up their large, mid-May push, some said it was a waste of resources since the bison would be forced to leave the park for want of grass.

Mike Mease with the Buffalo Field Campaign said high water in important river valleys is putting much of the bison's food underwater.

"I've never seen the Madison River so flooded. Basically, I don't think they have anything to eat in there, and they're turning around," said Mease, who monitors bison management from West Yellowstone.

But Zaluski said the continued presence of bison outside the park has more to do with scale.

"I don't think the lack of green-up was a critical factor here. ... We haven't gotten bison (leaving the park) in large, large numbers. A majority of those 600 to 700 bison have actually stuck," he said. "The problem is, even if you're 90 percent successful with 700 bison, you're still going to have 70 bison coming out."

Last year, hazing ran until June 19. The effort, which generally involves several agents on the ground as a helicopter flies overhead, can prove costly. It is funded by a $600,000-plus federal appropriation, secured in part by Montana's congressional delegation. Zaluski said the department is operating within that budget, despite the extended operations.

Daniel Person can be reached at dperson@dailychronicle.com or 582-2665.

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