Thousands of elk and bison on the National Elk Refuge north of Jackson will start receiving supplemental feed today.

On Monday, biologists assessed remaining natural forage and found it depleted to “negligible levels” on the south end of the refuge. To prevent the animals from dispersing in search of new food, officials decided to distribute alfalfa.

The feed start day falls five days after the average start, Jan. 26, ­from the past decade.

“We expected that it probably would occur this week,” elk refuge manager Steve Kallin said. “We have been conducting surveys and monitoring the amount of forage and snow conditions for weeks, and it started to move toward the threshold where we would want to start feeding.”

About 5,000 elk and 700 bison were tallied Monday by refuge biologist Eric Cole and Wyoming Game and Fish biologist Doug Brimeyer.

Cole and Brimeyer observed 5 to 10 inches of snow on the refuge. It was a mix of powder and dense, crusty snow, a refuge biological update said.

“I don’t believe we’ve had significant icing yet,” Kallin said of the snow conditions.

Lack of forage is the primary driver for the decision to feed, not snowpack, Kallin said. In 2012, due to drought, growth of grasses and forbs was 19 percent below the 15-year average.

Feeding doesn’t start in full force right away, Kallin said.

“Their stomachs need to able to adjust to a higher nutritional feed, so we gradually introduce them to that feed,” he said. “Otherwise, if it was abrupt, they could get bloated and die.”

Feed rates vary, Kallin said, but on average, each elk receives about 8 pounds of pellets each day and each bison about 20 pounds a day.

“We adjust that based on the environmental conditions,” he said.

The refuge’s new irrigation system delayed the need to feed, Cole said. About 2,600 acres of the 3,200-acre-capacity system were harnessed this year, adding an estimated 13 percent more forage than would have grown under natural conditions.

“It certainly helped us, but it’s difficult to say it added X number of days,” Kallin said. “There are so many variables involved with when you start to feed — the number of animals, snow, the temperature, when they arrive.”