Irrigation tends mainly to be a "drouth year" practice in Iowa. Results and needs vary with the moisture situation, type of soil and drainage. Here's a report on the results so far from 7 years of irrigation tests.
Shrader, W. D.; Johnson, Howard; Ericson, Laurel; and Gray, Don
"What Can You Expect From Irrigation?,"
Iowa Farm Science: Vol. 14
, Article 4.
Available at: http://lib.dr.iastate.edu/farmscience/vol14/iss8/4