Soybean meal is the most widely used protein supplement in horse feeding with excellent results. Soybean meal is an excellent source of protein, especially for the young growing horse.
It contains the highest level of lysine of the plant protein concentrates used in horse feeding. It is even higher in lysine than dried skim milk, which itself is an excellent source of protein for the young foal.
Soybean meal contains either approximately 44% or 48% protein; the 48% soybean meal is prepared by removing the hulls. In practice, however, it is usually fed in combination with other protein sources. Properly processed soybean meal by the hydraulic, expeller, or solvent method has about the same feeding value. Overheating soybean meal destroys lysine, whereas underheating soybean meal makes its methionine less available.
For maximum nutritional value, therefore, soybean meal needs to be properly heat-treated and processed.
Whole uncooked soybeans should not be fed to horses since they contain a trypsin inhibitor, which prevents the enzyme trypsin from digesting protein . There may be other factors in uncooked whole soybeans that require heat treatment before use by horses.