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.
On average, 2,100 head are over-wintered after the annual fall roundup and about 600-700 calves are expected each spring. In 2008, the herd reached its targeted over-winter size of 2,100 which is the optimal herd size to maintain on the 23,304 acre bison pasture.