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Agronomy Day 2010

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Tour C

Corn Yield Variability on Central Illinois Farms

Gary Schnitkey
Gary Schnitkey
Professor, Farm Management
Department of Agricultural and Consumer Economics
University of Illinois
217 244-9595

Illinois Farm Business Farm Management (FBFM) data were used to determine expected levels and variability of corn yields on central Illinois farms.  So doing provides estimates of yield risk faced in central Illinois.

To accomplish this, yield data for central Illinois farms were obtained from Illinois FBFM.  For each farm, rolling five-year averages from 1999 through 2008 were calculated. For a farm, the rolling five-year average for 1999 was calculated using yields from 1994 to 1998, the 2000 rolling five-year average was calculated using yields from 1995 through 1999, and so on.  Then, each years’ yields were divided by their respective rolling five-year averages to calculate a “yield-relative-to-five -year-averages”.  To illustrate, take a farm having a 2008 yield of 207 bushels with a rolling five-year average yield of 186 bushels.  This farm has a 2008 “yield-relative” of 1.12, indicating that the 2008 yield is 12 percent higher than the five-year average yield. 

From 1999 to 2008, yield-relatives averaged 1.07. This suggest that yields average 7% higher than their corresponding 5-year average.  Average yield-relatives vary across years, with above average yielding years resulting in higher yield-relatives and below average yielding years resulting in lower yield-relatives. The highest average yield-relative of 1.18 occurred in 2004, a year in which yields were above average across much of the state.  The lowest yield-relative of .88 occurred in 2005.

The 1.07 yield relative can be used to estimate the most likely yield for an upcoming year.  For example, suppose a farm had a five-year average yield of 195 bushels from 2005 to 2009.  An estimate of the average yield in 2010 is 208 bushels (208 = 195 bushels x 1.07 yield-relative).

The distribution of yield-relatives across time and across farms provides indications of yield variability.  From 1999 to 2008, .2% of the time yield-relatives were below .5, indicating that it would be reasonable to expect yields to be below 50% of the rolling five-year average about .2% of the time.  Yield-relatives between .5 and .6 occurred .3% of the time, between .6 and .7 occurred 1.2% of the time, and  between .7 and .8 occurred between 3.1% of the time.  The sum of all percentages below .8 equals 4.8%.  Hence, these historical estimates suggest that yields will be 80% of the five-year average about 4.8% of the time.  A farm with 195 bushel five-year average can expect 4.8% of their yield outcomes to be below 156 bushels (195 bushels x .8).

Yield-relatives varied by year.   In most years, less than 5% of the farms had yield-relatives below .8.  In only three years were yield relatives above 5%.  The highest percent of farms occurred in 2005, with 27% of the farms have yields less than .8 of their five-year average.  The next highest year occurred in 2002, with 17% of the farms having yield-relatives below .08.  In 2001, 7% of the farms had yield-relatives below .8.

In summary, historical data suggests that farmers can expect their yields to average 7% higher than their five-year average yield.  About 5% of the time, yields will be 80% of the five-year average.

Percent of Yield Relative to Five Year Yields Below .8 by Year
Percent of Yield Relative to Five Year Average Fall Within a Range
Agronomy Day 2010