What is protein degradability?

Question: What is meant by the term protein degradability and what does it mean for beef cows?

Protein degradability is a term used in ruminant nutrition and therefore applies only to ruminant animals like cattle, sheep, and goats, etc. To get a handle on this concept let’s define crude protein (CP), and how it is measured, and then relate it to ruminant nutrition. The CP content of feed is measured by analyzing for the total nitrogen (N) in the feed and multiplying by 6.25 (because, on average, protein is about 16% N; that is, 1/0.16 = 6.25). For example, a hay sample with 2% nitrogen would have about 12.5% CP (2% x 6.25 = 12.5%). Therefore, you can see that there must be other “stuff” in the CP than just protein. Sure, there’s true protein and remnants (amino acids, peptides, etc.) in the CP, but there’s also non-protein nitrogen such as found in DNA, RNA, and the like that gets lumped in with the true protein for ruminants. That’s good though because ruminants can use the non-protein nitrogen to satisfy the N requirements for the rumen microbes the same way they use N from true protein. It is also what makes it possible for us to feed non-protein nitrogen like urea with good success to satisfy part of the protein requirements of ruminants.

Now on to more details of protein degradability: Ruminally Degradable Protein (sometimes referred to as Degradable Intake Protein) is the part of the protein that is broken down in the rumen by the rumen microbes. The rumen microbes need nitrogen to make protein in their cells. When the microbes degrade the feed protein in the rumen, some prefer NH3 (ammonia) as their nitrogen source (it came from the degraded proteins), other microbes like amino acids and peptides (building blocks of proteins). It all depends on the specific species of rumen microorganisms, and it is highly related to what types of feeds the microbes like. Satisfying the rumen microbes’ requirement for N from protein makes it possible for the rumen microbes to effectively break down forages and other feeds. Ruminally Undegradable Protein (sometimes referred to as Undegradable Intake Protein) is that portion of the CP that escapes rumen degradation and is digested in the small intestine (also called bypass or escape protein).

What does this all mean? In a practical sense, a cow’s metabolizable protein requirement is met in two ways, from microbial protein and bypass protein. Yes, as the microbes reproduce and as they die off, they move through the digestive tract into the small intestine and are digested there to provide protein and energy for the animal. In addition, bypass (escape) protein makes up the other part of the protein requirement. In some diets, the protein from the rumen microbes makes up 50% or more of the protein needed by the animal. Quite amazing, eh?

On another note, I would be remiss if I didn’t mention one more item relative to protein and its utilization. At times proteins can be chemically bonded to carbohydrates which can influence how well the protein is digested and used by the animal. This is usually observed when forages are harvested too wet (and heats) or other feeds are processed in a way that uses additional heat and causes bonding to the carbohydrates. When this happens adjustments my need to be made when determining the amount of protein available to the animal. Hay that is not sufficiently dry when baled will heat and give off an odor of ammonia. Ammonia is a clear sign that the CP is breaking down and of course this is a detrimental protein loss in addition to the harmful mold from the wet hay. All part of the big picture of forage and feed quality.

There you have it, the finer points of protein degradability. Pretty exciting for ruminant nutritionists, important for cattle producers. Please let me know if I can assist you with your cattle feeding programs.

— Don Llewellyn

is the WSU Lincoln County Extension Director in Davenport. He can be emailed at don.llewellyn@wsu.edu