Heat Stress, Plant‐Available Soil Moisture, and Corn Yields in Iowa: A Short‐ and Long‐Term View

The weather of the 1980s has been very dramatic relative to crop production in a large part of the USA, providing both excellent and stressful years. Because of this, and also because of general concerns and predictions of more stressful times in years to come, a study was conducted to identify weather factors that (i) significantly affect corn ( Zea mays L.) production in Iowa, and (ii) were available in long‐term weather records. Regression analyses identified heat stress and plant‐available soil moisture as the two most important weather‐related variables affecting corn yields in central Iowa. Heat stress was defined as the monthly or seasonal accumulation of the number of degrees each day when maximum temperatures exceeded 86 °F (30 °C). The regression model used included terms representing technology (time), heat stress in July and August, 1 July plant‐available soil moisture in the top 5 ft of soil, and an interaction term between the last two variables. The model accounted for 90% of the variation in corn yield between 1954 and 1988. Time trend accounted for 62% of the yield variation and weather factors were 28%. When 1 July plant‐available soil moisture was below normal (approximately 7 in.), heat stress in July and August caused greater yield reductions than when plant‐available soil moisture was above normal. To compare our present climate with previous years, a seasonal heat‐stress variable was computed for 1900–1988. This revealed periods of both benign (1958–1975) and hostile (1900–1957) characteristics relative to expected and realized corn yields. Runs of stressful days each season showed similar patterns of season‐to‐season variability. The 1 July plant‐available soil moisture trends for central Iowa clearly show that the benign period mentioned was also one of adequate levels of crop growing season soil moisture. It is not possible to predict future weather, but this work identifies two variables (heat stress and 1 July plant‐available soil moisture) that, over a long period of time, reveal considerable interannual variability. The late 1950s, the 1960s, and the early 1970s were very favorable for corn production in central Iowa, as well as in other parts of the Midwest. These data show this to be an unusual and benign time period when compared to other time periods of this century. The ups and downs of the 1908s may be closer to our expected weather.