Meeting the protein requirements of your cattle is a cornerstone of a well-balanced nutrition program. Not only is protein one of the most expensive nutrients to supplement, but it is also one of the most important from a performance perspective.
Consider, for example, that protein requirements increase during the latter stages of pregnancy and with the onset of lactation. With growing calves, protein requirements increase with expected performance level. Failure to supply adequate protein not only limits performance but can also compromise the health of the animal. With this column, I want to explore the theory behind protein nutrition of cattle. Next article I’ll apply this theory to practical protein supplementation strategies.
First, it is important to realize that an animal does not have a requirement for “protein” per se. Rather, it requires amino acids, which are the building blocks of the various proteins found in plants and animals. There are 22 amino acids, 10 of which are essential, with the remainder classified as non-essential. The difference is that non-essential amino acids can be synthesized by an animal and thus are not required in the diet, while essential amino acids cannot be synthesized and thus need to be supplied in the diet. This is true of pigs, chickens, cattle and even people. Examples of essential amino acids include lysine, leucine, methionine and cysteine.
While it is true that cattle require the 10 essential amino acids for maintenance and productive functions, they differ from other animals in that they can meet their essential amino acid needs without a defined dietary supply. This ability stems from rumen fermentation of feeds consumed by the animal. Specifically, rumen bacteria ferment or break down the carbohydrate and protein fractions of feedstuffs and use the resulting release of nutrients for their own growth, much of which comprises protein. Rumen-bacterial protein consists of a well-balanced blend of essential and non-essential amino acids. These bacteria eventually pass out of the rumen with undigested feed and are subject to digestion in the small intestine. As such, the essential amino acid content of bacterial protein serves as a source of essential amino acids for the animal.
It is also important to understand that rumen bacteria can synthesize amino acids (both essential and non-essential) from basic building blocks such as dietary sources of carbon, nitrogen and sulfur. In other words, they do not require a dietary source of essential amino acids for their growth. This is an important concept as it means that cattle can be fed relatively poor sources of protein (i.e. straw, poor-quality hay) and the rumen bacteria, through their growth, can convert these poor sources of essential amino acids into high-quality microbial protein. This can in turn be used to meet the animal’s essential amino acid requirements.
The rumen bacteria’s ability to synthesize amino acids from “scratch” is the reason we can supplement cattle with sources of non-protein nitrogen such as urea and still meet their essential amino acid requirements. This symbiotic relationship between the rumen bacteria and the animal provides ruminants with a unique ability to meet essential amino acid needs and has important consequences for cattle feeding.
The rumen microbial population also consists of various species of protozoa and fungi. However, these organisms — while a source of high-quality protein — are not as important to the animal as the rumen bacterial population.
A second source of amino acids to the animal is that from undigested feed protein, the amount of which varies with feed source. Ultimately, both protein fractions leave the rumen and pass to the small intestine where amino acids are digested and absorbed. Absorbed amino acids are used for both maintenance and productive functions.
While it is true that cattle require amino acids for protein synthesis, it has been difficult to establish amino acid requirements for various classes of cattle. As a result, most nutritionists formulate diets based on the concept of metabolizable protein. This term refers to protein that is available for absorption in the small intestine and consists primarily of microbial and undigested feed protein.
To understand how we get to metabolizable protein, we need to start with the concept of crude protein. Crude protein is simply the nitrogen content of a feedstuff multiplied by a constant. This constant (6.25 per cent) represents the nitrogen content of common protein sources. When analyzing for crude protein, feed test laboratories typically measure the nitrogen content of a feedstuff and multiply the resulting value by 6.25 per cent to get the crude protein content of the sample.
The crude protein content of a feedstuff or ration can be subdivided into rumen degradable (RDP) and undegradable (RUP) fractions. Rumen degradable protein is protein or non-protein nitrogen (i.e. urea) that is degraded in the rumen and serves as a nitrogen source for rumen bacteria. Rumen undegradable protein is that undigested feed protein that bypasses the rumen and is potentially available for absorption in the small intestine.
The microbial protein and the undigested feed protein that leave the rumen form the vast majority of metabolizable protein available to the animal. Next month we will examine how we can use the concept of RDP and RUP to meet metabolizable protein requirements.