From Muppet Wiki

Jim Henson produced an ad for Cloverland Dairy in Baltimore in 1965 and 1966. Three versions of the ad were produced -- one 60-second, one 20-second, and one 10-second. In the commercial, "Sing a Song of Cloverland", the Cow dances through a moving farmland background, singing a Cloverland song previously used in radio ads. The puppet Cow lip-synched to the pre-recorded track, which included the line, "If you don't own a cow, call Cloverland now, it's North-9-222!"

The moving background was an early use of Chroma key.