Long-term variations in seasonal weather conditions important to corn production in Illinois

An analysis of long-term records of corn yields and seasonal weather conditions in Illinois found major temporal shifts and important spatial variations in the types of seasonal weather conditions, expressed as the normality of temperature and rainfall conditions, that create positive and negative impacts on yields. Nineteen different types of corn-weather seasons (May-August) occurred during 1901-1997, and 9 of these seasons accounted for most of the high corn yields (top 20 of the 97) and 8 seasonal types produced most low yields (lowest 20 values). Assessment of the years with either high or low yields revealed (1) some corn-weather seasonal types were uniformly distributed throughout the century; (2) others were unevenly time distributed, some occurring only in early decades and others only in the last few decades; and (3) certain weather types varied regionally. The yield responses to certain seasonal types varied over time, being good early and not so later, or vice versa. The findings indicate that time changes in farming practices, corn varieties, and agricultural technology have altered how certain types of growing season weather affect corn yields. Regional differences in yield outcomes from a given set of weather conditions reflect varying soil and climate differences across Illinois. The spatial and temporal differences reveal the importance of using weather-impact results to define seasonal weather extremes. The kinds of seasons creating high yields predominated during 1901-1910 and 1961-1997, and most seasons creating low yields were concentrated between 1911 and 1960. Comparison of the 1901-1997 temporal distributions of yield extremes (high and low) with the temporal distributions of cyclone passages and the incidence of ENSO conditions that affect growing season weather conditions revealed a generally good relationship. The shifting temporal responses to various kinds of seasonal weather conditions during the 20th century reveal some of the difficulties facing efforts to project realistic and detailed impacts in agriculture from altered future seasonal weather conditions.