Crop physiology and production (a.k.a. agronomy) entails a complicated mix of factors dependent on a multitude of site specific situations driven by micro and macro environmental conditions.  Prediction becomes a messy business because growing conditions are never duplicated.   This creates a situation whereby agronomists and farmers fall back on a more predictable science, mathematics, to create a wonderful little number known as “the average” or “the mean.”  The mean is clean, fast, generally derived from good research, and it can be communicated with little further explanation.  We love it.  Of course around every mean are things called a standard error and range, which we don’t like so much because they often cry for explanation. 

      All of the above brings us to the relationship between corn grain and silage yields.  For many years the standard was 7 bushels of grain for each ton of silage......clean and fast.  The number was used to convert silage yields to grain and vice versa.  Farmers, FSA directors, extension agents, and scientists all had the number committed to memory.  Further, it was used extensively as a starting point to value corn silage each fall by multiplying 7 times the value of corn grain
(e.g. 7 x $2.00 = $14.00 per ton).  

      Several years ago, Joe Lauer, UW Extension Corn Agronomist, reported that the relationship between corn grain and silage yield had increased to nearly 8 bushels per ton based on data collected in 256 research plots in 1997 and 1998.  This seemed reasonable given that hybrids had changed over the years and the relationship had probably changed as well.  Hence, for the past five years a new “average” was used and communicated as needed.  Not the end of story.

        Lauer continued his pursuit to verify this relationship and began crunching even more numbers (1858 total field plots harvested as both silage and grain) and looking at how factors such as grain yield, silage moisture, and year to year variability impacted the relationship.  You guessed it........a new average with lots of variability.  The numbers are presented in Tables 1 and 2.  What we see is a shift back toward our original number of 7, but with a large amount of variability depending upon year, grain yield, and silage moisture.  Here is a sampling of the variation noted: 

·         At 65% silage moisture, grain equivalent ranged from 5.2 to 7.4 bushels per ton depending upon grain yield.

·         For 150 bushels per acre corn, grain equivalent ranged from 6.3 bushels per ton for 70 percent moisture silage to 8.4 bushels per ton for 60 percent moisture silage.

·         Depending upon the year, grain equivalents ranged from 6.4 (1999) to 9.4 (2003) at the 150 bushels per acre yield level. 

        What all of this means is that estimating the relationship between corn grain yield and silage yield with an overall “average” for all years and situations will probably make us wrong most of the time.  The accuracy of such a prediction can be improved by taking into account silage moisture and real or estimated grain yield, especially if they are drastically outside of normal ranges.

How do we account for year to year to variation?

Take an average.......