Protein is not Protein is not Protein

Protein is often the first limiting requirement when selecting diets and designing supplementation strategies for cows and growing cattle. Age and stage of production impact how much protein an animal requires. Understanding the different types of protein can help tailor supplements to meet protein requirements economically and effectively.               

Crude protein (CP), the nitrogen content of the feed, is the initial measure of protein most commonly evaluated in feedstuffs. Crude Protein can be further divided into rumen degradable protein (RDP) or rumen UNdegradable protein (RUP). Rumen degradable protein is the protein that is available for rumen microbes to use. These microbes work in the rumen to break down and ferment the forage consumed by the cow. Feeds with a higher percentage of RDP than RUP (as a percent of crude protein) include forages, soybean meal, and non-protein nitrogen (NPN) sources, such as urea and ammonia. These NPN sources are typically used in protein supplements 30% or higher in protein concentration.

Rumen UNdegradable protein is the remaining portion of the crude protein protected in the rumen and available for breakdown in the lower digestive system. Feeds with a higher portion of RUP include distiller’s grains, corn gluten meal, corn grain, and blood meal.

Microbial crude protein (MCP) comes from the turnover of microbes in the rumen resulting in a source of protein digested in the lower digestive tract. As the microbes reach their lifespan, they become protein to be broken down into amino acids for the animal to absorb.

Crude protein and RDP requirements are often stated, rarely is an RUP or MCP requirement discussed. Instead, it will be represented as a metabolizable protein (MP) requirement. Metabolizable protein is the combination of RUP and MCP, or protein broken down post-ruminally. This differentiation is sometimes explained as the needs of the microbes (RDP) and the needs of the animal (RUP + MCP=MP). Metabolizable protein is essential to all ruminants but plays a critical role in immature animals for growth, development, and achieving desired gain.

For example, a 1,300 lb cow in mid-gestation can maintain on a diet that provides 7.1% CP daily. This is her lowest CP requirement in the production cycle and should include both RDP and MP. If she is consuming a forage-based diet adequate in protein and total digestible nutrients (TDN), a mature cow can turn over her rumen microbe population at a rate that meets her MP requirement. In contrast, a 1,000 lb replacement heifer mid-gestation lacks the rumen capacity to produce enough MCP to meet her required MP justifying supplementation of RUP. This is not unlike formulating diets for classes of growing calves.

In recent history, distiller’s products have provided an economical RUP supplement to growing cattle. With changes in price and supply, evaluating other feeds is warranted, depending on supplement goals. The table below lists protein sources and their composition of RDP and RUP. Pricing feeds based on nutrient content (i.e., $/lb of RUP) is the best economical way to compare feedstuffs relative to the feeding goals. If selecting a feedstuff lower in CP or RUP than what is normally used, expect animal performance to reflect that decreased nutrient content. However, the lower CP or RUP feedstuff can make economic sense within resource and management constraints.