Rumen degradable protein (RDP)

Rumen degradable protein (RDP) is needed to feed the rumen bacteria and ensure an adequate supply of microbial protein but rumen undegradable protein (RUP) is also important.

Microbial protein is the highest quality protein available and we want to produce as much as possible. Rumen undegradable protein passes through the rumen unchanged and some of it can be digested in the small intestine. It compliments the microbial protein that is produced and is necessary for cows producing at higher levels of production. If we can match the RDP and RUP we maybe can feed less protein % than is commonly practiced.

Certainly cows producing at high levels need some RUP source in their diets. Also a balance of RDP, RUP, and nonfiber carbohydrates is needed. The nonfiber carbohydrates are sugars and starches and supply energy for microbes in the rumen. They complement the RDP, which must be converted to microbial protein to be used by the cow. Below is an indication of how the 2001 NRC model predicts utilization of the RDP and RUP in cows producing 77 lbs. of milk per day.

Excess RDP and RUP both will result in wastage of nitrogen and excretion into the environment. Notice that only 45% of RDP is absorbed as amino acids from microbial protein. The rest is mainly excreted in the urine. Adequate energy is needed in the rumen for efficient nitrogen utilization to occur. This energy mainly comes from sugars and starches plus some from fiber digestion. Of the RUP, 85% is absorbed. This emphasizes why RUP is important for efficient protein utilization. The amino acid profile of the RUP is also important. Therefore, a proper balance of RDP, RUP, and ruminally available energy is necessary for efficient nitrogen utilization.

By Charles C. Stallings
Extension Dairy Scientist, Nutrition
Virginia Tech Cooperative Extension