A balanced dairy ration best matches nutrient supply to requirements. This implies both supply and requirements need to be accurately defined.

Large improvements have been achieved over the last decades to refine our assessment of protein supply and requirements of dairy cows; we progressed from crude protein to a combination of rumen-degradable and rumen-undegradable protein.

Nevertheless, neither rumen-degradable protein nor rumen-undegradable protein is a direct assessment of the real supply to the cow. The former represents the nitrogen available to the rumen micro-organisms, whereas the latter only represents a fraction of what is available to the animal.

More recently, complex rumen sub-models have been developed integrating many parameters (e.g., rumen-degradable protein, rumen-undegradable protein, energy, rate of passage, intestinal digestibility) to estimate the flow of protein being digested and available to the animal – these are the metabolizable protein.

Supply of metabolizable protein is a better predictor of milk protein yield than crude protein intake. Therefore, balancing diets based on metabolizable protein rather than crude protein is a first step in improving the efficiency of utilization of nitrogen.

However, metabolizable protein supply is an aggregate of individual amino acids: 20 amino acids are used to synthesize all the proteins in the body, including tissues, hormone, enzymes, milk, etc.

The synthesis of protein from amino acids can be compared to the writing of very long words. With the 26 letters of the alphabet, we can write any word; in comparison, the cow can synthesize any protein with the 20 amino acids.

However, just like the letters in a word, each amino acid needs to be in the right order at the right place. Of the 20 amino acids, 10 cannot be synthesized, at least not sufficiently, by the animal and should be provided by the digested protein.

These are called the essential amino acids and include arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Currently, we focus on these essential amino acids to balance dairy rations.

The models developed to estimate metabolizable protein supply also offer the opportunity to estimate the flow of digestible essential amino acids.

Three questions then rise: Why should we consider individual amino acids to better define supply and requirements? How do we best estimate the utilization of the supply of metabolizable protein and amino acids towards milk production? Would we gain in efficiency by balancing for individual amino acids rather than balancing for metabolizable protein?

Why should we consider individual amino acids?

At the tissue level, the free amino acids are used by the cells to build proteins; therefore, the essential amino acids need to be supplied in adequate quantity. However, if amino acids were always present in the same proportions, we would not need to partition metabolizable protein supply into individual amino acids.

A comparison of the profiles of three essential amino acids from microbial protein and different feed ingredients clearly indicates a disparity in their amino acid composition (Figure 1).

Therefore, large variations in the concentration of amino acids in duodenal protein have been reported. For example, lysine varied between 9.7 and 18 percent of essential amino acids, whereas methionine varied between 2.2 and 7.1 percent of essential amino acids in duodenal flow of protein.

This clearly indicates the digested protein cannot be considered as a homogenous entity and needs to be characterized in terms of its constituent components, the amino acids, to really assess what is supplied to the dairy cow.

On the other hand, the different protein secretions requiring amino acids for their synthesis can also vary in their proportion of essential amino acids.

For example, threonine concentration is known to be elevated in the metabolic fecal protein secreted in the feces, averaging 13.4 percent of essential amino acids, whereas it is only 8.9 percent in milk (Figure 1). Therefore, the total requirement of amino acids will depend on the quantity of each type of protein secreted or gained by the dairy cow.

Indeed, many studies have indicated it is the supply of essential amino acids and not total metabolizable protein which drives milk protein yield.

With sufficient knowledge, rations could be balanced for individual essential amino acids and, if the requirement of each essential amino acid is met, then there should be no need to balance for metabolizable protein. In fact, this is currently what is being done in pig nutrition, where they can prepare synthetic diets including large proportions of essential amino acids.

This has even led to a shortage of total nitrogen for de novo synthesis of non-essential amino acids, which can be alleviated through supply of non-protein nitrogen in the diet.

Such a shortage of non-protein nitrogen should not happen in dairy rations, as rumen-degradable protein has to be supplied sufficiently to support microbial growth. Indeed, we always have to keep in mind that when balancing a dairy ration, we have to feed first the rumen microflora with sufficient rumen-degradable protein and degradable energy to optimize microbial protein synthesis.

The microbial protein is the less expensive source of protein and has a good profile of essential amino acids, except for histidine, which might be a little bit low. Then we need to complement the microbial synthesis with appropriate undegradable protein.

How do we estimate utilization of supply?

Actually, it is assumed in most of the American feeding systems the efficiency of utilization of metabolizable protein is fixed, i.e., that the losses are directly proportional to the metabolizable protein supply.

The NRC (2001) applies a fixed efficiency factor of 67 percent for the conversion of metabolizable protein supply into milk protein. The CNCPS uses a different but fixed efficiency factor for each amino acid. Therefore, these models estimate a fixed return of increased protein or amino acid supply into milk protein; however, few biological systems work in such a linear fashion.

We have proposed an efficiency, which was decreasing as the supply of metabolizable protein and amino acids was increasing.

European feeding systems (e.g., the French and Scandinavian systems) are now using a variable efficiency related to the supply of both protein and energy. Feeding systems including a variable efficiency predict more realistically the protein yield to be expected from a ration, and the American systems are moving towards this trend.

Would we gain in efficiency?

Balancing a ration for individual amino acids rather than metabolizable protein is like playing Scrabble when you could ask for specific letters rather than receiving a handful of random letters. You would need fewer letters to write the desired words and would have fewer useless letters in excess.

Protein fraction represents an expensive fraction of the ration and, furthermore, the cow has to eliminate the amino acids in excess through synthesis of urea, which is excreted in urine and has a negative impact on the environment.

The economic and environmental impacts of balancing dairy rations based on revised recommendations and using the factorial approach and variable amino acid-dependent efficiency have been evaluated in three different Canadian contexts. The three Canadian contexts were the Maritimes, Central Canada and the Prairies, with respective average milk production per cow per year of 8,608, 9,102 and 9,198 kilograms.

Assuming no effect on milk protein yield, the net income (Canadian dollars per kilogram of energy-corrected milk) was calculated with diets balanced only for metabolizable protein using NRC (2001) or balanced with revised recommendations only for the three most likely limiting amino acids, histidine, lysine and methionine.

The net income (Canadian dollars per kilogram of energy-corrected milk) increased from 0.079 to 0.102, 0.195 to 0.212 and from 0.210 to 0.229 when diets were balanced for individual amino acids rather than metabolizable protein, for each region respectively. Also, the nitrogen balance (grams nitrogen per kilogram of energy-corrected milk) decreased from 12.2 to 11.9, 13.7 to 12.4 and 13.8 to 13 for each region respectively, implying an overall better usage of nitrogen on the farm.

Therefore, balancing for these three essential amino acids would have global positive impacts on dairy farms, increasing net income and reducing total crude protein supply and nitrogen excretion – and this across a variety of feeding systems.

Conclusion

The decreased efficiency of transfer of absorbed amino acids into milk protein with increased protein supply is directly related to increased catabolism of amino acids by different tissues.

We still need to determine the key factors involved in this loss of efficiency of transfer of absorbed amino acids into milk protein. So far, the supply of metabolizable protein or amino acids and the supply of energy seem to be involved in this efficiency.

A challenge for dairy farmers and nutritionists will be to make a transition, as there will be no more a single set of requirements of metabolizable protein and amino acids, but these will be determined in relation to the energy supply, as currently done in pig nutrition. Furthermore, sufficient provision of rumen-degradable protein to maintain rumen health and optimize microbial protein synthesis will always remain relevant.  

Hélène Lapierre, Roger Martineau and Daniel R. Ouellet are with Agriculture and Agri-Food Canada. Simon Binggeli and Doris Pellerin are with Université Laval’s Department of Animal Science.

References omitted but are available upon request. Click here to email an editor.