The bushel measurement was not defined in terms of cubic feet, but is currently considered to be about 1.25 cubic feet in volume. To facilitate the trading of grain, the USDA created weight standards for each grain so that grain could be weighed to determine the number of bushels rather than trying to make volume measurements. Corn was assigned a bushel weight of 56 pounds, while soybeans and wheat were assigned bushel weights of 60 pounds. Different grades of each grain have different standard test weights. No. 2 yellow corn has a standard of 56 pounds per bushel while No. 3 yellow corn has a lower weight.
Test weight is determined on each load of grain sold by weighing a known volume of the grain.
The seller of grain with test weight greater than the acceptable range is usually not rewarded for a superior product. Varieties of a crop often vary in their inherent test weight. When this happens the starch molecules inside the grain are prevented from the natural process of shedding absorbed water molecules that allows the grain to shrink to a normal size. During germination, oil, starch, and protein are digested to provide energy and molecules to produce a new seedling. This process leaves small voids inside the grain.