I would agree that a census in the midst of war would be impractical. However, if you look at the history of Missouri you will find that there was a large population drain just before and during the war and a large gain after the war. This is especially true for the area bordering Kansas, including Benton County, whose residents took part in the Border War before the Civil War.

In my ancestors township, in Benton County, the 1860 census showed only 422 residents in 70 households; the 1870 census for the same area contained 799 residents in 154 households. There were only 5 families that were in both the 1860 and the 1870 census. Meaning 65 families moved out and 149 families moved in between 1860 and 1870. This is a 93% turnover and the population almost doubled.

According to one history source:
“During the war, an estimated one-third of Missouri's population moved out of the state. … The governor and state legislature responded to the population decline by creating the Missouri State Board of Immigration in 1865 and embarked on an active campaign to attract immigrants from other states as well as from foreign countries. By 1870, Missouri experienced a 45.6 percent population increase…”

Source citation: Donald L. Stevens, Jr.; A Homeland and A Hinterland. Omaha, Nebraska: National Park Service, 1991, chapter 5. Digital image; National Park Service (http://www.nps.gov/history/history/online_books/ozar/hrs0b.h...: accessed 16 April 2009).