Visitors from across the world cheered from behind wire fencing as whooping horseback riders chased the thundering, wooly giants across hills and grasslands in Custer State Park. Bison and their calves stopped occasionally to graze on blond grass and roll on the ground, their sharp hooves stirring up dust clouds.

"How many times can you get this close to a buffalo herd?" said South Dakota Game, Fish and Parks Secretary Kevin Robling, who was among 50 riders herding the animals. "You hear the grunts and the moans and (see) the calves coming and running alongside mamas."

Each year Custer State Park holds one of the nation's few bison roundups to check the health of the bison and vaccinate calves, park Superintendent Matt Snyder said.

As many as 60 million bison, sometimes called buffalo in the U.S., once roamed North America, moving in vast herds that were central to the culture and survival of numerous Native American groups.

They were driven to the brink of extinction more than a century ago when hunters, U.S. troops and tourists shot them by the thousands to feed a growing commercial market that used bison parts in machinery, fertilizer and clothing. Because bison were essential to Native Americans, the U.S. government also encouraged hunters to kill the animals as a way to force tribes to leave their homelands and move to reservations. By 1889, only a few hundred remained.