A total of 1,110 district entries was made in the 12 fields of the 1938 Iowa Corn Yield Test. These were divided into three groups on the basis of the indicated seed grown by the entrant in 1937; Regular Open-Pollinated, Regular Hybrids, and Experimental Hybrids.
Zuber, Marcus S. and Robinson, Joe L.
"1938 Iowa corn yield test,"
Bulletin (Iowa Agricultural Experiment Station): Vol. 34
, Article 1.
Available at: http://lib.dr.iastate.edu/bulletin/vol34/iss379/1