Soybean hulls are a byproduct of soybean processing for soybean oil and soybean meal. During processing, soybeans are rolled or cracked to break the whole bean into smaller pieces and the hulls are separated by an air stream. Hulls are usually toasted to destroy their urease activity and ground to the desirable particle size.
The soybean hull is high in fiber (73 percent) and low in protein (9.4 percent). The protein is highly degradable, while the cell wall is low in lignin and highly digestible. Ground hulls are often sold as soy mill feed. This feed contains some meal so the protein content may equal 12 to 14 percent. Soy hulls are very palatable and are typically used to increase bulk in rations of fine texture.They are a good source of digestible fiber, but not so good in terms of effective fiber. Their maximum incorporation in dairy cattle rations should be 20-25% of the dry matter.
Soybean meal is the most common plant protein supplement. It is the remaining product at the end of the oil extraction process from soybeans. Oil can be extracted mechanically or via the utilization of solvents. The use of solvents is the most efficient and common technique resulting in a meal that contains 48% crude protein. Typically, millfeed (ground soy-hulls) will be blended with this meal to produce the more common 44% crude protein soybean meal. There is also an expeller or old processed soybean meal that contains 42% crude protein and 5% oil.
Soybean meal is highly palatable and has a moderate level of rumen undegradable protein (RUP). The most limiting amino acid to milk protein synthesis in soybean meal is methionine.
Very palatable feed that is easily incorporated into many different feeding systems as a topdress, part of a total mixed ration, or in a grain mix. It can be fed either raw or roasted. Raw soybeans should not be fed in diets containing urea because of their urease activity. Roasted whole soybeans contain more bypass (undegraded) protein and the urease and trypsin inhibitors are denatured.
Large variation exists in the nutritive value of beans that have been roasted. These differences are explained by differences in temperature, time of heating, moisture of the seed at the start of roasting, and how beans are handled after they are roasted. Drum-roasted soybeans should exit the roaster at approximately 295°F and be held without cooling for an additional 30 min.