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Evaluating and comparing protected amino acids for dairy cattle

Stephen B. Blezinger for Progressive Dairyman Published on 02 February 2016

With every passing day as we work to understand the nutrition, physiology and genetic potential of the dairy cow, we grow closer to formulating and implementing more precise diets.

Ongoing research, model development, ingredient analysis, improved skill in feeding management and so on leads us to better understand the complexities of the rumen and the animal. This includes balancing the inputs needed to optimize the products of fermentation as well as those ingredients and nutrients that avoid this process for more direct absorption at the intestinal level.



For much of the dairy industry, use of rumen-protected amino acids (RPAAs, primarily methionine [Met] and lysine [Lys]) has become fairly standard, especially in those markets where components are part of the milk valuation and pricing structure. Over recent years, numerous products have emerged showing effectiveness as supplements for either Lys or Met in the cow’s diet. One of the challenges for dairy nutritionists, given the cost of these products, is to find the products that are not only effective but also the most cost-effective.

One thing to keep in mind is that prior to implementing amino acid (AA) supplementation, some basic, foundational ration balancing principles need to apply. These should include but are not limited to:

  1. Feeding high-quality, fermentable feed ingredients and effective fiber to maximize production of VFA and microbial protein.
  2. Feeding appropriate but not excessive levels of rumen degradable protein (RDP) to meet rumen bacterial requirements for AA and ammonia, and to allow for maximum carbohydrate digestion and synthesis of microbial protein.
  3. Feeding high-Lys protein sources or supplements and an rumen-protected (RP) Lys supplement to reach a level of Lys in diet metabolizable protein (MP) that meets the optimal concentration per formulation model as closely as possible.
  4. Follow up by feeding RP-Met supplement in amounts required to achieve the optimum Lys-Met ratio in diet MP. Then fine-tune for optimized milk protein concentrations.
  5. Limit rumen undegraded protein supplementation based on what cow production says.

These are base guidelines for inclusion of RPAA sources in the diet per the formulation models, but remember that cow response is still the best indicator of diet formulation effectiveness.

Several of the computer models currently available can predict varying levels of Lys and Met needed to balance dairy diets based on each version’s set of equations. So depending on the model, the ingredients, the accuracy of ingredient analyses and the formulator, the exact amount of supplemental RP-Lys or RP-Met required to meet the requirements will vary.

When considering RPAA sources, another variable is the actual availability of the Lys and Met in the product itself at the intestinal level. The various companies that produce RPAAs protect Met and Lys by using different coating or protection technologies to limit the ruminal degradation. These protection technologies include fat coatings, fat matrices and polymer coatings. In some cases, AAs complexed with certain trace minerals have also been used as a source of RPAAs, although this is not cost-effective.


To function properly, the AA must be protected from degradation in the rumen but be correctly sensitive to pH and enzymatic activity upon passing through the abomasum and into the small intestine to be rendered available for absorption at the intestinal lining. The challenge here is that the protection technologies may function differently under different dietary conditions (fiber contents, fermentation rates, rumen pH, etc.). For instance, it might be observed that a given RPAA may not “behave” the same in a diet composed of high levels of forages as compared with a diet where a higher starch content is fed.

Comparisons between products have created significant discussion and argument. Observations have been made that a “standard” assay is necessary to objectively compare the net availabilities of a given RPAA source. For instance, there are two fairly common techniques for assessing RPAA availability to the cow. One of these includes the use of Dacron bags, where the product is introduced into the digestive tract of the animal and provides a simple measurement of net-in, net-out. While this is a reasonably simple and inexpensive process, it does not provide a clear picture of how well the given RPAA will change circulating and tissue levels of the AA in the animal.

Another method is a blood plasma technique, which seeks to measure the amount of AAs actually absorbed by the animal as related to the amount fed. This technique, while potentially more accurate, is very expensive. Reported studies using this method also, in general, have used very limited numbers of cattle and thus are subject to animal variability.

Other methods have been suggested and researched, but only time will tell if the industry can develop a standard, accepted means of analysis to assist nutritionists in objectively comparing the different sources of RPAAs on the market. Until that point, it is a matter of reviewing each company’s product data, assessing their research and determining which one best fits the specific diet. Ultimately, it becomes a matter of determining which product(s) the cow responds to most effectively. Unfortunately, at this point, there are no simple answers.

Finally, there’s always that final pesky detail of cost. While we always seek that supplement that appears the most cost-effective, we are reminded that least cost is not always best cost. The best solution is that product that creates the best net return (milk volume, components, other performance) per dollar invested. Again, not a simple answer.

Hopefully in the reasonably near future, nutritionists can look forward to the development of enhanced evaluation techniques that can simplify and expedite the comparison of these supplements.  PD


Stephen B. Blezinger is a nutritional and management consultant with Reveille Livestock Concepts in Sulphur Springs, Texas. Email Stephen B. Blezinger.

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