Weather and crop yields in the Midwest have exhibited wide fluctuations during the past 25 years. Sea surface temperatures (El Niņo and its counterpart, La Niņa) have been related to or blamed for these weather abnormalities. Other climatic variations seem to be affecting these yields after a 20 year period of regular and invariant growing conditions. Corn yields have been affected by drought conditions and excessive water conditions. This study attempts to quantify the effect of extreme phases of the Southern Oscillation on corn yields in the Corn Belt. It, also, attempts to describe other variations in local climate that are affecting yields.
Yield, weather, and El Niņo related (southern oscillation index - SO) data beginning in 1900 were assembled to determine if significant temperature, precipitation, or yield relationships could be ascertained. Midwestern monthly high temperature, precipitation, soil moisture and corn yield data were coded according to the SO. The SO groupings were summers where the SO reached < - 0.8 (El Niņo like - low phase), > 0.8 (La Niņa like - high phase), or in between. Regional monthly t-tests of mean low and high phase responses were calculated for high temperature and precipitation throughout the year. Forty years of Iowa growing season soil moisture data are available for comparison to yield responses.
Annual yield data were grouped as > 10% or < 10% of the expected yield, or in between. During the summer when the SO was in the low phase (high phase), there was a statistical tendency for corn yields to be higher (lower) than expected, respectively, for all Corn Belt states studied, except Missouri, Nebraska and Kansas. During the low (high) phase of the SO, much of the Corn Belt received more (less) rainfall in July, August, and September. At the same time heat stress (periods of high temperatures greater than 86o F), were generally lower (higher) during the low (high) phase of the SO. Increased (but not excessive) precipitation and lower temperatures are associated with good corn yields by reducing the stress on the corn during a potentially stressful time of the growing season.
While mean responses were good, there were several outliers.
Some outliers and further explanation of yield variability comes from two other climatic variations. Moist soil conditions from the winter and early spring countered some of the heat stress occurring during dry periods of high phase summers. A regional soil moisture model confirms this finding. Some poor yields during low phase events occurred because of excessive rainfall. Four excessively wet early parts of the year have caused significant yield reductions in Iowa and nearby states in the last 25 years.