It’s that time of year again when the combines will be threshing, the yield monitors blinking, the weigh wagons weighing, and the calculators calculating to determine yields and coffee shop bragging rights through the upcoming winter months.  Most extension agents receive a number of phone calls each fall  with requests to convert wet bushels to dry bushels or estimate the number of bushels that would fit in a given bin, silo, wagon, or garage (the latter generally only applies in those “bumper crop” years).  I thought that perhaps now was a good time to stop and contemplate the question, “What exactly is a bushel?”  Further, what is the relationship of bushel yield to grain moisture, test weight, and that pesky little number referred to as “shrink.” 

        A bushel is a volume measurement.  Specifically, a volume bushel is the equivalent of 1.244 cubic feet (a figure of 1.25 cubic feet is often used).  Looking at it another way, there are 4 pecks or 32 quarts in a bushel.  Test weight is a measure of the weight of grain (usually expressed in pounds) per volume bushel.  Each different type of grain has a standard bushel weight at a specific moisture content.  These are: corn (shelled) – 56 lbs. at 15.5% moisture, soybean – 60 lbs. at 13% moisture, oats – 32 lbs. at 14% moisture, wheat – 60 lbs. at 13.5% moisture, and barley – 48 lbs. at 14.5% moisture. 

        Given these relationships, volume calculations can be used to get a reasonable estimate of the number of bushels that can be held in a storage unit.  Similarly, grain weight can be used to estimate bushels and is probably the most accurate method if adjusted for moisture.  The question sometimes arises as to how you figure the equivalent shelled grain bushels of ear corn.  In this case, a volume bushel is figured as 2.5 cubic feet.  

        Grain moisture and test weight are related from the standpoint that as moisture increases, test weight decreases.  For example, corn at 20% moisture will have a test weight that is about 2 pounds lower than the same corn dried to 15.5% moisture.  It doesn’t matter whether the drying is done naturally in the field or artificially in a bin.  We often see higher test weight figures from field plots in years where corn has been early planted and fall conditions are favorable for drying.  One reason why test weight increases as grain dries is that dry kernels pack together more easily than wet ones.  Additionally, as moisture decreases, the kernels shrink and this allows for more kernels to fill a volume bushel. 

        Shrink is the weight loss that occurs to corn during a mechanical drying process.  The shrink factor is usually cited as the percentage weight loss for each point of moisture removed.  When considering water weight loss only, the shrink factor is constant for any given final moisture [determined by dividing 100 by (100 minus the percent final moisture)].  For example, the shrink factor for a final moisture of 15 percent is 1.176.  Total water loss for corn dried from 25 percent down to 15 percent would then be 11.76 percent (10 points removed x 1.176).  The other factor that goes into determining a total shrink is handling loss.  There will typically always be some weight loss to corn after it is dried that is not fully accounted for by water loss alone.  Actual handling loss will vary from one operation to another but should be less than 1 percent of the total weight loss.  Typically, handling loss increases as initial grain moisture increases.  

      Quoted shrink factors from grain buyers differ for several reasons.  First, they may use a different final moisture to determine water shrink.  Second, and probably the biggest factor, is that they use different values for handling loss when determining their final shrink factor(s).