Soybeans are grown because oil can be extracted and marketed as vegetable oil or blended to make biodiesel and the high protein soybean meal byproduct can be fed to livestock. However, it is the storage proteins and their composition that determine protein compositional quality.
Soybeans, at 13 percent moisture contain about 18 to 20 percent oil and 34 to 36 percent crude protein, though those ranges can vary considerably across varieties and climatic zones. A bushel of soybeans weighing about 60 pounds contains 44 pounds of meal, 11 pounds of oil, 3.5 pounds of hull and about 1.5 pounds of water.
Estimating crude protein is an easy and inexpensive measurement using NIR (Near Infrared) technology. It is widely used for establishing protein price levels for meal. However, it is not the most accurate in predicting the amino acid levels of soybean meal. The trend in the future will be to focus more on amino acid levels in meal and less on crude protein levels.
Soybean meal is considered the gold standard when it comes to high protein (hi-pro) meal with a good balance of amino acids, not a perfect amino acid balance, but better than most other meal sources that come from plants. The feed industry wanted a hi-pro meal, which contains 47.5 to 48 percent crude protein. The popularity of soybean meal in swine and poultry feeds is due to its high concentration of protein (44 to 48 percent) and its excellent profile of highly digestible amino acids. Nutritionists use this value as an index of the amount of individual amino acids available in meal, which feeds directly into their formulation software.
Soybean meal doesn’t have a perfect blend of amino acids as it tends to be low in the sulfur-containing amino acids, methionine and cysteine. However, it is a rich source of lysine, tryptophan, threonine, isoleucine, and valine – the amino acids that are deficient in corn and other cereal grains commonly fed to pigs and poultry.
Soybean crude protein is primarily comprised of storage proteins in the seed. These storage proteins contain 88 different proteins and fall into one of four categories: albumins, globulins, prolamins, and glutelins (based on solubility and extraction). Of the 88 proteins, the two most common storage proteins in soybeans are glycinin and β-conglycinin, both in the globulin category. These two proteins account for over 80 percent of the total protein in the seed.
However, the amino acid compositions of glycinin and β-conglycinin vary and this affects their nutritional value in animal diets. Glycinin and β-conglycinin are deficient in sulfur amino acids; therefore, there is a need to understand these storage proteins and how they are produced in the seed in order to improve sulfur-containing amino acids and soybean proteins. The trend in the future should focus more on improving sulfur amino acids and less on storage protein levels.
In 2012, the Illinois Soybean Association funded a whitepaper titled “Importance of Storage Protein in Soybeans,” by Stephen O. Opiyo, Daniel J. Davidson, Eliot M. Herman and Gladys A. Opiyo. Click here to download this whitepaper.