Nice item and quite an unusual topic for a slogan cancel. I guess back in 1914, things such as a "Corn Expo" warranted a special cancellation. I note that the US Post Office had to purchase these slogan cancels for a total cost of $38, which was probably quite a sum in 1913-14 especially when they would have only seen use for little more than 60 days and then discarded.
Thought you might be interested in this news article link for the event (zoom into 4th column):
http://texashistory.unt.edu/ark:/67...1/m1/5/zoom/