Test Weight as a Feed Grain Quality Factor
Livestock Update, December 2003
Allen Harper, Extension Animal Scientist Swine, Tidewater AREC
Several factors can impact quality and value of feed grains. Such characteristics as moisture content, foreign material, and mold and mycotoxins are all relevant. In addition, grain test weight is a simplistic but useful assessment of feed grain quality.
Test weight is actually a measure of the density or weight per unit of volume of a grain at a standardized moisture level. In U.S. grain merchandizing channels this is typically expressed in pounds per bushel. Standards for test weight are set by the USDA Grain Inspection, Packers and Stockyards Administration (GIPSA). The following table gives minimum test weight standards for USDA quality grades in selected feed grains. Other factors such percentage of foreign material, broken kernels and heat damaged kernels can also alter the USDA quality grade. The complete set of grain quality standards can be found at the USDA GIPSA internet site (www.usda.gov/gipsa/reference-library/standards/standards.htm).
Minimum Test Weight Standards (Lbs./Bushel) for USDA Grain Quality Grades
Test weight is important because of the direct relationship this factor has on energy content and feeding value of the grain. Feed grains of high test weight will have a high percentage of large, plump kernels with a greater proportion of starch rich endosperm and a lesser proportion of bran and hull. Consequently, higher test weight grains tend to have average crude protein content but a higher metabolizable energy content. Lower test weight grains tend to have slightly higher crude protein percentage but reduced endosperm size results in substantially lower metabolizable energy concentration. In swine and poultry feeding the most important contribution of feed grains is to provide dietary energy. Therefore high test weight grains have greater feeding value than lower test weight grains.
An illustration of this was shown in trials by Bob Harrold and co-workers at North Dakota State University. Linear improvements in growth rate and feed efficiency of finisher pigs were observed as grain test weight of barley used in a barley-based diet increased from 39 to 41 to 44 lbs. per bushel. This trend held true when the barley-based diets were fed in meal form or in pellet form. Corn-based diets were also used in the experiment and the same trends were observed. As test weight of corn used in the diet increased from 46 to 49 to 51 lbs. per bushel, growth rate and feed efficiency improved.