How Protein Source Affects Protein Metabolism in Ruminants

By Warren Rusche and Madeliene Nichols

Distillers grains have become the standard supplemental protein source for growing and finishing beef cattle. They are readily available in the Midwest, usually priced attractively, and contain appreciable amounts of crude protein. While it is common to balance beef cattle diets based on the crude protein system, it does not paint an accurate picture of what is occurring inside the animal, leading to gaps in nutrition and economic losses. To overcome these issues, many producers and nutritionists have adopted the metabolizable protein system, which partitions the protein requirements into those of the ruminal microorganisms and the host.

Rumen microorganisms require peptides, amino acids, and ammonia to grow and to break down feed nutrients. These inputs can be supplied by ruminally degradable protein (RDP) from feedstuffs or ammonia from non-protein nitrogen (NPN) can also be used by the rumen microbes. Most cattle diets meet or exceed microbial RDP requirements, but in instances where RDP is low, the rumen microorganisms will recycle urea to meet their requirements.