The government program was aimed at killing free-roaming bison infected with brucellosis, a disease that affects fertility in cattle. But the culling, combined with hunting, disease and starvation brought on by a harsh winter, caused the herd's population to drop precipitously from 4,700 last November to 2,300 now, CNN reported Tuesday.

Hunting and wildlife management programs alone accounted for more than 1,600 of the reduction, CNN said.

Bison were hunted to near extinction in the 19th century and Yellowstone has the last uninterrupted wild herd in the lower 48 states, though there are a few other wild herds.

The U.S. Agriculture Department estimates half the bison in Yellowstone are infected with brucellosis. Wildlife managers say a spread of the disease to cattle would have a ripple effect in the beef industry.

But Mike Mease of the Buffalo Field Campaign supports the halt to the culling program.

"There has never been a slaughter like this of the bison since the 1800s in this country and it's disgusting," he told CNN.

Wildlife officials limited the culling program to infected bison during the winter and shelved further programs until this summer.