The Nature Conservancy. Protecting nature. Preserving life.™

Each morning at sunrise during the Annual Bison Roundup, 300-500 bison are herded into the corrals for their health checkup at the Tallgrass Prairie Preserve near Pawhuska, OK.

Consisting of almost 40,000 acres near Pawhuska in Osage County, the Tallgrass Prairie Preserve is the largest protected remnant of tallgrass prairie left in the world! Since 1993, the Conservancy has proven successful at restoring this fully-functioning portion of the tallgrass prairie ecosystem with the use of free-roaming bison. The herd started with 300 bison in 1993 and continues to thrive despite the drought.

“Keep in mind that the Tallgrass bison herd is not given any dietary supplements or hay – just the grass that grows and the water that flows,” Tallgrass Prairie Preserve Director Bob Hamilton said. “We over-wintered 2,117 bison in their 23,304 - acre unit and 674 bouncing baby bison were born in 2012!”

This year’s calving rate was 72.9% which was above the 19-year average of 71%. Of the 674 babies, 333 of them were female and 341 were male. The largest calf weighed 445 lbs (6-8 months old), while the smallest calf (1-2 months old) weighed 85 lbs.

The largest bison that came across the scales at this year’s roundup was a 1,770 lb. bull that was born in 2006. During the annual roundup, all heifer calves are vaccinated against brucellosis. All keeper animals are vaccinated for several bovine diseases and treated for external and internal parasites. For research and record-keeping purposes, each individual bison in the herd is identified with an ear-tag transponder.

A total of 620 bison from this year’s roundup were sold. Bulls are sold at 6-7 years of age, since after this they tend to become more aggressive and dangerous. Cows are sold at 10-12 years of age.

Preserve staff expect 600-700 calves to be born in the spring of 2013.

The chart below shows the growth of Tallgrass Prairie Preserve bison herd since 1994, including the number kept on the preserve over the winter, as well as the number of calves born the following spring. The number of calves is determined at the fall roundup, when each animal is examined, weighed, and inoculated. The fall roundup, which takes about a week, is the only time the bison are herded and confined on the prairie. The roundup permits the Conservancy to perform various scientific studies and ensure the health of the herd.