South Dakota cowboys and cowgirls rounded up a herd of more than 1,500 bison Friday as part of an annual effort to maintain the health of the species, which has rebounded from near-extinction.

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 occassionally 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.”

Custer State Park holds the nation’s only Buffalo Roundup once a year 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. By 1889, only a few hundred remained.

“Now, after more than a century of conservation efforts, there are more than 500,000 bison in the United States,” said South Dakota Gov. Kristi Noem, a horseback rider who took part in the roundup. “The Custer State Park bison herd has contributed greatly to those efforts.”

The park’s herd began with 36 animals bought in 1914. A state ecologist estimated the park can currently sustain about 1,000 bison based on how snow and rain conditions affected the grasslands this past year, according to Snyder.

The other 500 or so will be auctioned off, and over the next week, officials will decide which bison will remain and which will go. About 400 calves are born in the park each year.

“Each year we sell some of these bison to intersperse their genetics with those of other herds to improve the health of the species’ population across the nation,” Noem said.