Amino acids (AA), essential nutrients for dairy cows, cannot be considered optional feed additives. When AA levels in the ration are properly balanced, milk production, the cow’s reproduction and health each benefit.

Should inadequate AA levels be present, however, milk production suffers along with the cow’s reproduction and health.

Historically, diets have been defined in terms of their crude protein (CP) level. Today, though, we think in terms of amino acid levels. Twenty amino acids exist in varying levels in feedstuffs and serve as the building blocks of protein.

The diet’s CP level is a measure of the nitrogen (N) concentration in the feed X 6.25 (6.25 lbs of total protein per pound of nitrogen). The CP level does not correlate with AA levels.

AAs do contain N, but so do other feed components such as urea and ammonia – neither are true proteins but are counted as CP due to their N content. Therefore, judging the quality of a diet by its CP can be misleading – working with the ration’s AA composition is far more accurate.


Cows require the AAs to build various proteins. Proteins are built from 20 AAs, 10 of which are essential, meaning that they must be provided in the diet because the cow cannot synthesize them.

If the diet does not provide these 10 essential AAs, the cow experiences a deficiency. In comparison, simple-stomached animals, such as poultry and swine, are very sensitive to AA deficiencies. Their diets have been carefully formulated to provide the required AAs in the proper proportions for quite some time.

The more complex digestive system of ruminants offers advantages and disadvantages. Feeds ingested by the ruminant undergo fermentation in the rumen, a large fermentation chamber, by a massive microbial population.

Millions of rumen microbes break feed protein down into peptides, AA and even into ammonia. The microbes then use the diet’s AA and non-protein N to feed themselves, thrive and reproduce. Many of the microbes will move with other rumen contents into the small intestine.

Microbial protein

The AA profile of rumen microbial protein matches the AA profile of milk protein far better than the proteins in any feedstuff. The microbes’ lifespan in the rumen is short, allowing the microbes (microbial protein) to flow into the lower part of the digestive system.

Once in the lower digestive tract, the microbial protein is broken down into its individual AAs. These, in turn, are absorbed and used by the cow as the building blocks for the protein in milk and meat.

In a sense, we feed the rumen, and the rumen feeds the cow. Some protein is broken down to AA and non-protein N upon reaching the rumen. This is defined as rumen- degradable protein (RDP). The remaining feed protein flows out of the rumen intact and is known as rumen-undegradable protein (RUP).

Even though the cow’s microbial protein has the perfect AA profile, the rumen’s daily output of microbial protein is insufficient to satisfy the requirements of modern dairy cows. Nutritionists, therefore, provide supplemental RUP to assure the cow is properly fed.

The disadvantage of the rumen digestive system is that it is very complex to understand and control. Nevertheless, we must aim to maximize rumen function, to exploit its fermentation capacity and its ability to digest feed ingredients – goals not possible with simple-stomached animals.

Once we achieve maximum rumen function, we need to supplement the diet with the right RUP to assure that the flow of protein to the lower digestive tract provides the proper AA profile to satisfy the cow’s needs.

The total protein flow available to the cow is called metabolizable protein (MP). We now recognize that the diet must supply “rumen protein” – or RDP – and “cow protein” – or RUP. The AA profile of the RUP must complement the AA profile of the microbial protein in order to avoid an AA deficiency.

This can be done by feeding feeds with high RUP or by supplementing the diet with specific rumen-protected AAs.

Balancing AA levels

For years, nutritionists have known that methionine (Met) and lysine (Lys) are the most limiting AAs in North American diets. Much of the research on balancing AAs is focused on making sure Met and Lys are provided in sufficient quantities.

In addition, several commercial sources of Met and Lys are available in different forms to protect the Met and Lys from rumen fermentation. They can be used to balance the AA profile of the MP.

When balancing the AA of the cow’s diet, the objective is to deliver the right AA profile in the MP such that the cow does not experience a deficiency. The question is not: Should you balance the AAs of the diet? The question is: Are you balancing them properly?

If we feed a diet with unbalanced AAs, the cow will not stop producing milk but her milk production becomes less efficient and N is wasted.

Each protein has a very specific AA makeup. This is what makes each protein unique and different from the other. Casein, for example, is a unique protein found in milk and important for cheese manufacturing.

The cow produces casein from individual AAs transported via the blood system to the mammary gland. The amount of casein the cow produces directly relates to the amount of AAs available. Casein production stops if a required AA is not available.

This AA then becomes a limiting AA: its unavailability prevents the cow from expressing her potential milk production.

When this happens, the other AAs present are in excess. The surplus AAs are wasted as they cannot be used in casein synthesis. Wasting AAs is expensive for dairy producers and dairy cows because energy is spent to dispose of them. In addition, N is excreted in the urine. Limiting N excretion helps protect the environment.


Balancing AA levels in the ration maximizes the cow’s ability to efficiently utilize protein and maximizes the total milk protein produced. Research trials and field practice have shown increases in the true protein output from cows fed diets balanced for AAs. In addition, cows are healthier.

In today’s environment, balancing the AA levels in rations is not an option. A previous lack of suitable tools to handle the complex interactions of ruminant protein metabolism limited the ability of nutritionists to balance AA levels in rations.

Today’s software, however, facilitates the computations required for formulating a ration to maximize N utilization.

AAs are essential nutrients, not additives. By ensuring cows receive diets properly balanced for AA levels, we further maximize the efficiency of milk production and cow reproductivity while also advancing cow health. PD

Dan Luchini Ph.D.
  • Dan Luchini Ph.D.

  • Manager - Ruminant Product Technical Services
  • Adisseo