An analysis was conducted to investigate climate influences and weather variability on annual agricultural yield and production value for four major crops (corn, soybeans, hay and winter wheat) common to the Midwestern United States. County-level data for crop production and price statistics are from the United States Department of Agriculture (USDA) National Agricultural Statistics Service (NASS) and are analyzed for the 1950-2006 period. Counties are aggregated according to U.S. state climate divisions, resulting in 47 sub-regions across six Midwestern states (Illinois, Indiana, Iowa, Kentucky, Missouri and Ohio).
A time series of crop yields in the United States displays a conspicuous rise in yield after the mid-20th century. This trend is collectively attributed to technological changes in farming practices, including the education and introduction of herbicides, pesticides fertilizers and genetically modified crops. To account for this trend, a linear regression analysis was run on annual yields for each crop and the resulting standardized residuals are used in subsequent investigations.
Detrended annual crop yields are correlated to monthly and seasonal atmospheric and oceanic teleconnection patterns prior and during the respective crop growing seasons. The strongest relationships are with corn and winter wheat yield to winter and spring teleconnection indices. For example, annual corn and winter wheat tend to display an increase (decrease) in yield during the positive (negative) phase of North Atlantic Oscillation (NAO). Furthermore, these findings also sometimes vary significantly temporally, often revealing an opposing relationship between periods before/after 1977 (a year linked to phase reversals in many of the teleconnection indices). Differences are also observed between the positive, negative and neutral phases. For instance, a stronger relationship with corn yield exists with the positive NAO and neutral conditions and is considerably weaker or non-existent with the negative NAO phase for some climate conditions. These relationships are further explained through a discussion of temperature and moisture fluctuations and prevailing synoptic conditions occurring during the related phases.