We all use the term crude protein to describe nutritional value without much thought about it, as if crude protein were something that has been around forever, like pi or I Love Lucy. But if you look carefully at the most recent National Research Council (NRC) reference tables for beef and dairy cattle, you’ll see that these tables go well beyond crude protein. They dissect protein into its component parts to describe feed quality and animal requirements. What’s going on here?
Here’s some background: Crude protein (CP) is called “crude” because its official assay analyzes only for nitrogen, not for real protein. This is a practical approach because it’s much easier to analyze for nitrogen rather than for true protein, and all proteins are composed of amino acids, which contain nitrogen. Since, on average, most proteins contain 16% nitrogen, we simply analyze for nitrogen, multiply that value by 6.25 (because 6.25 × 16% = 100%), and call the resulting number crude protein. For example, a feedstuff containing 2.1% nitrogen would be listed on a feed report as 13.1% crude protein.
But should the world of protein be so easily defined? In reality, protein nutrition for cattle is far more complex than a factor of 6.25. Lots of things happen to protein and nitrogen when animals consume feed: Microbes in the rumen and large intestine convert some of that protein into their own nitrogen compounds; nitrogen is recycled between generations of microbes; some feed nitrogen ends up in non-protein forms that are digested quite differently than real protein; and saliva, which contains nitrogen, effectively recycles excess metabolic nitrogen back into the rumen. The simple number for crude protein accounts for none of these conversions and, more importantly, it does not accurately predict the amount of true protein that a ration supplies to the animal for absorption.
Nutritionists have known about these issues for more than 70 years. The scientist H. H. Mitchell actually discussed some of them in a landmark paper published in 1929. Back then, however, we did not have the appropriate laboratory assays or enough understanding of rumen ecology to solve this problem. But over the past 35 years, this has changed, and nutritionists have developed the concept of metabolizable protein (MP) to evaluate protein nutrition. The concept is fairly straightforward: MP is the protein that an animal actually absorbs across the gut wall, while CP is the nitrogen that first enters the GI tract when an animal consumes feed. The trick is knowing how to convert CP to MP. This new protein system does just that.
First, crude protein is partitioned into two fundamentally different classes of protein:
- Class 1: Protein that passes intact through the rumen without destruction or alteration by rumen microbes. Since this type of protein is not degraded in the rumen, it’s called undegradable intake protein (UIP). (Get used to strange acronyms in protein nutrition. Sometimes I think that they were conceived by the same folks who defined an elevator as a “vertical access device.”) UIP is generally insoluble under rumen conditions and therefore cannot be fermented by the rumen microbes. The popular press often calls this type of protein bypass protein.
- Class 2: Protein that enters the rumen but then is degraded by rumen bacteria before it can pass into the lower tract. This type of protein is called degradable intake protein (DIP). The rumen bacteria first disassemble these proteins into amino acids and then use the nitrogen for their own compounds. Most of these bacterial compounds are true proteins, but some of these compounds are nucleic acids and cell wall molecules that are not digestible in the lower tract. Therefore, there is a certain amount of digestive inefficiency with all feed proteins classified as DIP, because some of their nitrogen is converted to indigestible compounds that are unavailable to the animal.
Second, each class of protein (UIP and DIP) has different digestibility characteristics in the lower tract. Remember, the digestibility of a protein is the amount that is actually absorbed across the intestinal wall into the blood. In general, the digestibility of UIP is approximately 80%. That 80% portion is called metabolizable feed protein. The remaining 20% of the UIP is composed of indigestible nitrogen compounds, such as nitrogen bound up in lignified plant cell walls or caramel polymers of heat-damaged protein (like Maillard products, which occur in wet hay after it heats).
In contrast to UIP, the digestibility of DIP is more complicated because of the actions of rumen microbes. Our calculation here is a two-step process:
- Step 1: First, we estimate the total amount of protein made by the bacteria, which is called microbial crude protein. This amount depends on microbial yield, which depends on the amount of fermentable material in the feed. Higher amounts of fermentable material usually support higher levels of microbial yield. The amount of fermentable material in a feed is conveniently estimated by the feed’s total digestible nutrient (TDN) value. In general, the yield of microbial crude protein is approximately 13% of the TDN, although this percentage can be smaller for diets containing lower levels of fiber.
- Step 2: Second, we need to adjust the microbial crude protein for the losses associated with bacterial action. We know that rumen bacteria use some of the DIP to create compounds that are not digestible in the lower tract, such as microbial nucleic acids and nitrogen compounds in bacterial cell walls (compounds like diaminopimelic acid [DAPA]). These indigestible compounds average approximately 36% of the total microbial protein, and we account for them by multiplying the value of microbial crude protein by 0.64. The resulting number is called the metabolizable bacterial protein.
And finally, we add the values of metabolizable feed protein (from UIP) and metabolizable bacterial protein (from DIP) to give us a number called the total metabolizable protein in the feed – the total amount of real protein absorbed by animals from the lower tract. This number is the MP value that you see in reference tables.
Whoa! That’s a lot of calculations. But look around; those MP values are showing up in more places than just reference tables.
