With more factors controlled and with greater knowledge about weather conditions, predicting yield levels on a statewide bases becomes more reliable. It seems that corn, soybeans,sorghum, and hay yields may be lower in 1967.
Duncan, E. R. and Shaw, R. H.
"Crop Yield Prospects for 1967,"
Iowa Farm Science: Vol. 21
, Article 6.
Available at: https://lib.dr.iastate.edu/farmscience/vol21/iss7/6