In the dairy nutrition business we routinely toss around the term “protein” when discussing diets for milk cows, heifers and calves. We refer to the protein content in the diet as a percentage of the total diet. We also discuss and advise the dairy farmer of the protein content of a pellet or a grist that comes from the feed mill. We recommend that the pellet should be raised from 20 percent protein to 22 percent so as to improve milk production. The all-important pricing point for a pellet or a grist is oftentimes based on protein content.

Since the 19th century chemists and biologists have known about protein and the essential role it plays in making life possible. Protein has been nicknamed “the building blocks of cells.” All organic matter has protein, from muscles to bones, from milk to blood and hormones to hair; protein abounds in organisms in many shapes and sizes. Every aspect of metabolism requires protein. Protein also makes up different portions of plant material, such as legumes and grasses, leaves and weeds.

The one single element that defines protein and sets it apart from all other organic substances is nitrogen. The analysis for protein content in feedstuffs was developed over a century ago by determining how much nitrogen is in a given sample. There’s a fairly constant ratio of nitrogen found in all proteins, therefore, by measuring the nitrogen content a protein value can be determined.

The term “crude protein” (CP) became the gold standard for many years when comparing the protein content of feedstuffs being fed to dairy animals. For many decades dairy rations have been balanced to a particular CP when being formulated to support a certain amount of milk production. However, during the middle of the 20th century, dairy scientists began to realize that proteins are very complex and that two diets having similar CP value behaved completely differently in a cow’s rumen.

Proteins are, in fact, made up of smaller units called amino acids. Every amino acid contains nitrogen. There are around 20 amino acids that have been identified, and they are, in turn, arranged in a multitude of ways to form proteins. Proteins differ from each other primarily by the arrangement and sequences of amino acids. Two dairy pellets manufactured at different feed mills may test for the same CP, indicating that they contain the same percentage of nitrogen ions, but they can be as different Protein Exposed as night and day when it comes to the amino acids they contain. This explains why two feeds testing the same CP will result in different levels of milk production.

Unlike the monogastric species, such as swine and poultry, in which proteins coming from the diet are broken down by stomach acids and enzymes into amino acids and absorbed in the small intestine, ruminants, with their complex digestive system, must first feed and nurture the billions of bacteria and other microbes that ferment feedstuffs in the rumen. Rumen microbes, which are proteins themselves, require nitrogen and amino acids to grow, reproduce and function. If the rumen microbes are not healthy and working efficiently, fermentation of feedstuffs in the rumen is compromised.

An important characteristic of protein coming from feedstuffs in a cow’s diet is how quickly the amino acids can be utilized by the rumen microbes. This is often described as “soluble protein” or “rumen degradable protein” (RDP). Every feedstuff has its own unique characteristics of RDP. The degradability of protein in the rumen is critical to a healthy and active population of rumen microbes.

The nitrogen needed by microbes comes in different packages. Fiber digesting bacteria require simpler forms of nitrogen, such as ammonia. Starch digesting bacteria require more complex chains of amino acids. The success or failure of a diet hinges on the types of protein and amino acids and the RDP component of that protein.

Rumen microbes serve a dual purpose in the health and metabolism of ruminants. They first function as the fermenters that break down carbohydrates into smaller substances that become the precursors for metabolizable energy for the cow after which they die. Secondly, those spent microbes become a significant part of the cow’s metabolizable protein that gets absorbed in her small intestine. This microbial protein has its own unique amino acid profile that happens to be the “ideal protein” for dairy cow nutrition.

Table 1. A comparison of the essential amino acid composition of body lean tissue, milk and ruminal bacteria with that of some common feeds.Data from Schwab, O’Connor and NRC (2001).


Microbial protein, unfortunately, supplies only about 50 to 60 percent of the necessary metabolizable protein to a highproducing dairy cow. The remainder of that protein must come from the feedstuffs that bypass the rumen and is often referred to as rumen undegradable protein (RUP). Essentially, no dairy cow feedstuffs can match the ideal amino acid profile of microbial protein. This remains the single largest challenge to improving protein nutrition to the high-producing dairy cow.

Researchers have determined that in order to be most efficiently utilized the RUP coming from the diet and absorbed in the small intestine must have amino acid profiles that closely match the essential amino acids found in rumen bacteria, milk and muscle. Taking special note of the amino acids lysine and methionine, highlighted in the table above, only a handful of feedstuffs commonly fed to dairy cows have the necessary lysine levels that match rumen bacteria.

Dairy cow diet formulations often fall short of meeting adequate lysine requirements. Byproducts such as dried distillers’ grains (DDG) or brewers’ grains, while they each have high levels of CP, fall woefully short on lysine. Commonly used meals such as canola and cottonseed also do not supply enough lysine for RUP. Wheat-midds, a byproduct with a respectable CP and a common ingredient in low-cost formulations, comes nowhere close to meeting amino acid requirements in milk cow rations.

Some dairy farmers make the mistake of overfeeding a feedstuff thinking it will help meet the amino acid requirement. This often results in the overfeeding of nonessential amino acids that doesn’t help the cow’s metabolism. Thus, the cows will need to use more energy to dispose of the excess nitrogen, which costs the dairy farmer even more money.

Forages fair no better for dairy diets. Even if a grass hay crop or silage tests 20 percent CP, it falls short of the all-important lysine needs. Alfalfa, which is a member of the legume family, comes closer, which is why it’s called the king of forages. Soybean meal, also a legume, is the king of proteins in the byproduct set. Animal-based byproducts, such as blood and fish meal, even though they have more desirable amino acid profiles, have fallen out of favor due to inconsistencies of production and high prices, and in the case of blood and meat have been rejected because of mad cow disease.

In recent years, more attention has been paid to histidine as being a third limiting amino acid in dairy rations. Note that most feedstuffs, except the forages, supply comparable histidine levels to rumen bacteria. Corn silage has obvious limitations in amino acid supplies for all three of the limiting amino acids: lysine, methionine and histidine. This explains why diets heavy in corn silage that are not supplemented with higher-quality proteins, such as soybean or canola meal, do not perform as well in milking herds.

Today’s dairy cow diets should no longer be balanced based solely on the crude protein content of feedstuffs or grain mixes and supplements. Proteins must be evaluated based on their amino acid profiles. The amino acid profiles of feedstuffs are also being more closely examined for their potential to limit nitrogen pollution. Diets that are more accurately balanced based on metabolizable amino acids have less nitrogen to waste. Feedstuffs, commodities and byproducts that are chosen merely for the content or the price of crude protein should be more closely evaluated by dairy farmers who are attempting to maximize feeding efficiencies and milk production while keeping excessive nitrogen out of the environment.

Cover Photo By Jevtic/