...which was taken in 1852 in West Canada (now Ontario).
(The related 1851 Agricultural census page)
About the 1851 census:
"While the census for New Brunswick and Nova Scotia was completed in 1851, Canada West and Canada East did not start their collection of data until the following year. Therefore, for Canada West and Canada East, what is known as the Census of 1851 officially began on January 12, 1852."
Is it possible that they (Duncan and Ellen) were married when census was taken (married 13 May 1852)? I have not been able to pinpoint precisely when this particular census page was enumerated.