The objective of this fact sheet is to explain how and why grain test weight is used in the grain market, and how producers can ensure acceptable test weights from their crop production program.
Bushel is a volume measurement for grain created many years ago by Celtic peoples (Scotland, Wales, Brittany, Ireland) to facilitate fair grain trade. The bushel measurement was not defined in terms of cubic feet, but is currently considered to be about 1.25 cubic feet in volume. Although grain is referred to in terms of bushels in the United States, it is referenced and traded on the basis of weight (tons or metric tons) throughout the rest of the world. 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. Some other examples are: Rye at 56 pounds per bushel, barley at 48, oat and fescue at 32, etc.
The test weight concept was developed many years ago by the grain trade as a means of accounting for the varying densities of grain caused by weather and/or production practices. When grain density is lower than the accepted standard (low test weight), more volume is needed to store and transport a given weight of grain, thus increasing storage and transport costs. Different grades of each grain have different standard test weights. For example, No. 1 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. If the weight is lower than the acceptable range, the sale is "docked" on a percentage basis. The seller of grain with test weight greater than the acceptable range is usually not rewarded for a superior product. The test weight is also used in quality standards for small grains like soft red winter wheat, barley, and oats, but soybean grades are not impacted by test weight. Other factors, such as cracked kernels, foreign materials, splits, and heat damaged kernels also contribute to grading standards. The details of how test weight differs between grades for corn and more common small grains can be seen in Table 1.
|Table 1. USDA Test Weight Grade Requirements for Shelled Corn, Soft Red Winter Wheat, Barley, and Oats.|
|U.S. Grade||Minimum Test Weight (lb/Bushel)|
|No.||Shelled Corn (lb)||Soft Red Winter Wheat (lb)||Six-Rowed Malting Barley (lb)||Two-Rowed Malting Barley (lb)||Oats (lb)|
|*Only four U.S. grades for malting barley and oats are defined.|
Varieties of a crop often vary in their inherent test weight. Two common causes of low test weights are:
- Grain is prevented from filling completely and/or maturing and drying naturally in the field due to a killing frost, hail, or insect damage. When this happens, the starch molecules inside the grain are prevented from the natural process of shedding absorbed water molecules that allow the grain to shrink to a normal size. Artificial drying with heat removes this excess water but the starch molecules do not shrink and grain size doesn't change appreciably, so test weight (density) remains low.
- Grain matures and dries naturally in the field but is sometimes rewetted by rainfall, dew, or fog causing the grain to initiate the germination process (precocious germination) before harvesting. 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. Although the grain may again dry in the field, the seed size does not change and the small voids inside the seed result in a decreased test weight. Maximum test weight is achieved when grain is harvested on the first dry-down and also at higher moisture. For example, the ideal harvest moisture for soybeans and corn is 16-19 percent moisture and 20-25 percent moisture respectively.
Grain is seldom sold at the standard moisture content (soybean—13 percent, corn—15.5 percent). When grain moisture content is greater than the standard, the grain weight is discounted to account for the extra moisture according to the formula: (100% - wet%) divided by (100% - dry%). For a sample of soybeans at 18 percent moisture, the calculation would be: (100 - 18) divided by (100 - 13) = 82/87 = 0.94.
Multiplying the weight of the wet grain by 0.94 will give the weight of the grain at 13 percent moisture. For example: 6000 pounds of soybeans at 18 percent grain moisture would become 5640 pounds of grain at 13 percent moisture (6000 X 0.94 = 5640).
While the 56 pound per bushel test weight for corn is based on a moisture content of 15.5 percent moisture, some grain purchasers will use a moisture value of 15 percent and retain the 56 pounds per bushel value for calculation. If grain is dryer than the standard, that same equation can be used to calculate the increased weight that should be credited to the seller, although that calculation is seldom made and the seller is not usually rewarded for the low moisture content. This calculation works for any grade of any grain for which a standard moisture content is specified.
Original author: Dr. Jim Beuerlein, Retired, Extension Agronomist. (Originally published in 2000.)