In 1982, the Iowa City Press Citizen claimed that that the first Corn Monument was erected in in 1913. For a Homecoming 2014 Iowa Now article, the date of 1919 was given as a potential starting date. Regardless, this image of the 1919 Corn Monument is the earliest one we found.
For the 1961 Homecoming the Daily Iowan did a reflection on the celebrations of 1926. The October 21, 1961 paper, on page 8, featured a photo of the monument lit up with the "I superimposed with an 'M' for Minnesota. It was built to allow cars to pass beneath it"