Written with contributions by Sydney Vanderhoff, former SDSU Extension Beef Nutrition Field Specialist.
Why does type of protein matter?
In case you missed it check out the article Understanding Protein in Feed. We are going to dive deeper into understanding rumen degradable protein (abbreviated as RDP) versus rumen undegradable protein (abbreviated as RUP). Not only is protein amount important, but so is the type of protein (RDP versus RUP).
- Rumen degradable protein is protein that is broken down by the microbes in the rumen and used for microbial growth.
- Rumen undegradable protein is protein that “escapes” the microbes and is digested in the small intestine.
The proportion of RDP and RUP in feedstuffs are not the same. It is important to understand the proportions in commonly used feedstuffs so the best-suited supplement can be chosen for the cattle. Protein in distiller’s grains is about 40% RDP and 60% RUP, making it a good RUP source. Protein in forages is mostly RDP, and urea is 100% RDP. Supplementation needs depend on the protein composition of the base diet. For example, growing calves on a forage-based diet often requires RUP supplementation to meet their metabolizable protein (abbreviated as MP) requirements due to forages being high in RDP.
Corn silage is a common feedstuff here in the Midwest for growing diets; however, it is also used in cow diets. Crude protein (abbreviated as CP) of corn silage ranges from 6.5 to 8.5% and is predominantly RDP. As ensiling time increases, the RUP content of corn silage decreases. Corn-silage based diets may need supplementation of both RDP and RUP to meet the protein needs of the growing calf.
Overfeeding Protein
Overfeeding RDP does not provide any additional MP once the microbial requirements have been met; whereas overfeeding RUP does provide more and more MP as the dietary supply increases. An example of this might be overfeeding distiller’s grain plus solubles, which is approximately 65% RUP (% of CP). When RUP is overfed relative to the MP requirements, excess protein is still absorbed. If excess protein (RDP) is not needed, it is broken down into urea, which is recycled in the rumen (to supply RDP), large intestine, or saliva. Once it has been recycled, and if RDP is still not needed, then it is excreted as urea in the urine. Cattle are very efficient at recycling nitrogen in the form of urea to ensure adequate RDP before excreting excess.
Manipulating RUP levels through heat
Ruminally undegradable protein levels in feedstuffs can be manipulated by feedstuff processing methods. Feedstuffs that are heated have an increased level of RUP when compared to the untreated version. A common feedstuff this can be seen in is soybean meal. There are multiple processing methods for soybeans to break down the whole bean into soybean meal, but for this example, the focus will be on mechanical or chemical processing. Mechanical processing aims to retrieve the oil contained within the bean by exerting extreme pressure on the soybean particles. This pressure creates heat, which alters the structure of the protein molecules, making them more ruminally undegradable. Chemically processed soybean meal often uses a solvent, like hexane, to remove the oils from the soybean meal. Since less heat is generated during this process, chemically processed soybean meals tend to be more ruminally degradable.
While heat treatment of feedstuffs can be utilized to increase the levels of RUP, it is important not to overuse heat within processing. A chemical reaction named the Maillard reaction can occur, which binds the protein and sugar molecules together in a ‘browning’ process. This reaction may increase palatability of some feedstuffs, but a decrease in digestibility within the small intestine of the animal may be seen due to this molecule change. In other words, the level of RUP will be increased, but may not be fully available for use by the animal.
Take Home Points or Considerations
- Understanding the proportion of RDP and RUP is important when determining the best feed option to meet cattle requirements.
- Overfeeding RDP above requirements has no additional value.
- Overfeeding RUP can be valuable and should be evaluated if it is cost-effective.
- Heat processing can increase the RUP levels of a feedstuff, but heat should not be overapplied, due to a reduction of protein digestibility.
