Here are three more references to "Round the Corn, Sally" as a corn-shucking song. The first is from 1848:

http://books.google.com/books?id=PCpKAAAAMAAJ&pg=PA287&dq=%22Round+the+corn,+sally%22&lr=&cd=23#v=onepage&q=%22Round%20the%20cor

I'm not sure on the dating of this one, which was published in 1894, but surely it is earlier than that:

http://books.google.com/books?id=2ncAAAAAYAAJ&pg=PA260&dq=Round+the+corn,+sally&cd=1#v=onepage&q=Round%20the%20corn%2C%20sally&f

And, from SLAVE SONGS OF THE UNITED STATES, by Allen, Ware and Garrison, published in 1867, a tune:

http://books.google.com/books?id=6frfZd0-1xkC&pg=PA68&dq=Round+the+corn,+sally&cd=4#v=onepage&q=Round%20the%20corn%2C%20sally&f=