It is suggested that you watch Video 01A and complete the exercise in the video before continuing with the lesson.

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The primary concern in this discussion is the influence of weather on crops and on crop production. Weather is the major uncontrollable factor that influences the development of crops. Initially, we will look at some of the historical trends in crop yield. We often hear about deviation from yield trend. This is a concern as deviation from midwestern yield trends during 1996-2007 was less than experienced during the previous 20 years and may be a phase of a repeated pattern. We attribute much of this to changing weather patterns and the interaction of the weather with the soils and the crops.

Yield Trends

The United States Department of Agriculture (USDA) publishes data with annual yields and trends. Typically, the USDA depicts yield trend as a straight line, averaging the yields of the past 30-years. The USDA trend estimate for corn, soybean, cotton, winter wheat, and rice was from 1970 through 1996 (Fig. 1.1).





Fig. 1.1 U.S. Corn, soybean, and wheat yield (bushels/acre); rice and cotton yield (pounds/acre); and linear trends . (data source:

Compare these yield trends with the Iowa state-wide average corn yields from 1972-2002. Notice the change in yield throughout the century. Study the figure and answer the discussion question that follows. Note: The U.S. Rice results are not typical of a trend; there seems to be a "stair-step" response over time. Try to rationalize the difference between corn trend and rice trend. Is winter wheat intermediate in trend response?

Fig. 1.2a Iowa state-wide average corn yield and trend. (data source:
Fig. 1.2b Iowa state-wide total corn yield historical record. (data source: