Why is non-protein nitrogen (NPN) often used in cattle supplements?
Beef cows consuming low-quality forages are almost always deficient in protein. Thus, supplements comprised of high-protein ingredients are recommended over high-energy supplements. The problem is that most high-protein feeds are expensive. Thus, we are constantly searching for less expensive alternative sources of protein. Urea and other sources of NPN offer that opportunity in ruminant animals.
What is NPN?
Nitrogen is a required chemical element of amino acids, which are the basic building blocks of protein. Urea is the most commonly used source of NPN in ruminant animal diets.
How does it work?
Animals, including cows, cannot directly use NPN as a source of protein. However, the microorganisms that live in the rumen (one of the compartments of the 4-part ruminant stomach) can convert NPN to protein. In brief, every time a ruminant eats, the population of microbes in the rumen grows and these “bugs” then digest the feed for the ruminant. However, to grow more microbes, the original microbes need protein. They are capable of manufacturing microbial protein from NPN. The microbial protein is then used to grow the new microbes. Once the feed is digested and passes from the rumen through the rest of the stomach and intestines, the microbes ride along with the digested feed and are in turn digested by the cow and used as a sourced of protein.
How much NPN can be used?
There are limits on the amount of supplemental crude protein that can be NPN. In other words, the rumen microbes need some natural protein to be incorporated into the microbial protein. Extensive research indicates that no more than 1/3 of the supplemental protein should be NPN. The other 2/3 should be natural protein. For example, in a 30% crude protein cake or lick tub, no more than 10% of the protein (1/3) should be NPN, and the other 20% should be from natural protein.
Why do we need to limit it?
If greater than 1/3 of the supplemental crude protein is from NPN, digestion of fiber and intake of the low-quality forage will be depressed. Typically, cow nutritional status (measured as body condition score) and performance (such as pregnancy rate) is similar to that from an all natural protein source up to 1/3 from NPN, but both BCS and pregnancy rates decline at higher levels of NPN. Another notable concerns is that palatability and consumption of supplements generally declines when NPN levels exceed about 40% of supplement protein.