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.
2,106 head were over-wintered after the fall 2007 roundup and about 550 new calves were expected in the spring of 2008. In the summer of 2008 the herd after calving reached about 2,650 head which is the optimal herd size to maintain on the 23,464 acre bison pasture.