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Westcott and Jewsion: Weather Effects on Expected Corn and Soybean Yields

2013 June 15

Corn Yield Model
A model for national corn yields was estimated over the past 25 years (1988-2012), thereby including both the 1988 and 2012 droughts. In addition to a trend variable, the model uses as explanatory variables mid-May planting progress, July weather (precipitation and average temperature), and a June precipitation shortfall measure in selected years. Including those variables helps explain previous yield variations and deviations from trend.

Corn plantings by mid-May are important for yield potential because that allows more of the critical stages of crop development, particularly pollination, to occur earlier, before the most severe heat of the summer. Earlier pollination is also generally associated with less plant stress from moisture shortages. Most of the corn crop develops in July, so weather in that month is included in the model, including variables for both precipitation and temperature.

Finally, while weather in June is important for development of the corn crop (and June typically has lower temperatures and more rain than July), effects of June weather are typically small relative to July weather effects. However, extreme weather deviations from normal in June can have larger impacts, as seen in 2012 and in 1988. To represent that effect, the model uses a measure of the precipitation shortfall from average in years when June precipitation is in the lowest 10 percent tail of its statistical distribution. The mid-May planting progress variable is based on weekly data from USDA’s National Agricultural Statistics Service and is prorated to May 15 from adjacent weeks’ results for years that the statistic was not reported for that specific date. The weather data is from the National Oceanic and Atmospheric Administration.

The planting progress and weather data used is for eight key corn-producing States (Iowa, Illinois, Indiana, Ohio, Missouri, Minnesota, South Dakota, and Nebraska). Those eight States typically rank in the top 10 corn-producing States and accounted for an average of 76 percent of U.S. corn production over the estimation period. An aggregate measure for the eight States for each of those variables is constructed using harvested corn acres to weight State-specific observations.

The effects of mid-May planting progress and July temperatures on corn yield are each linear in the model—for those variables, each unit of change has a constant effect on yield. Similarly, the June precipitation shortfall variable is linear for the years it is nonzero. However, the effect of July precipitation is nonlinear in the model to reflect the asymmetric response of corn yields to different amounts of precipitation above and below its average. That is, reductions in corn yields when rainfall is below average are larger than gains in corn yields when rainfall is above average. The model uses a squared term for July precipitation to represent that asymmetric effect. The estimated regression equation (table 6) explains over 96 percent of the variation in national corn yields during the estimation period (more than 91 percent of the variation around the equation’s trend).

Weather Effects on Expected Corn and Soybean Yields
Paul C. Westcott, USDA, Economic Research Service
Michael Jewison, USDA, World Agricultural Outlook Board

The model assumes a linear trend for corn yields with a weather forced variation.

Note that as of June 12, 2013: U.S. corn production for 2013/14 was estimated 135 million bushels lower to 14.0 billion bushels. Corn yields for the upcoming year were projected at 156.5 bushels per acre, a 1.5 bushel decrease from May’s estimate. The decrease in yields is due to delays in planting in some of the highest producing corn states.

The baseline trend projection for 2013 is 163.6 bushels per acre, if this information from May is accurate: The 2013/14 corn yield is projected at 158.0 bushels per acre, 5.6 bushels below the weather adjusted trend presented at USDA’s Agricultural Outlook Forum in February [Edit: Confirmed here]

But even the new, lower projection is well above the 2011 and 2012 yields.

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