Answer provided by Helene Lapierre and Daniel Ouellet, of Agriculture and Agri-Food Canada, and Lorraine Doepel, University of Calgary, who presented a paper on this subject at the Western Canadian Dairy Seminar last March. The following answer is excerpted from their paper.
Q: Do you feed protein or amino acids to make milk?
A: To optimize efficiency, nutritionists have the task to formulate rations that will best match nutrient supply to requirements. This requires the choice of a “unit” that will best define both the supply and the requirements. At the tissue level, it is clear that the raw materials used by the cells to build proteins are free amino acids. Therefore, an estimation of the requirements in terms of amino acids would theoretically cover exactly what is being used by the animal.
The determination of amino acid supply to the ruminant is, however, not an easy task. The net supply of amino acid to the dairy cow is determined by the amount of protein flowing and digested through the small intestine. Obviously, due to the extensive metabolism occurring in the rumen, assessment of the digestive flow of amino acids cannot be solely based on protein and amino acid intake, as is the case for monogastrics. Indeed, the net flow of protein at the entrance of the intestine is a combination of dietary protein that has by-passed the rumen and microbial protein synthesized in the rumen, whereas endogenous proteins only constitute a recycling of absorbed amino acids.
Complex rumen sub-models have been developed using rumen-degradable protein and energy, rate of passage, etc., to estimate the amount of protein being delivered to the site of digestion and, therefore, available to the animal: the now-called metabolizable protein. These models estimating metabolizable protein also offer the opportunity to estimate the flow of digestible amino acids…
Although the perfect system is not yet available to determine requirements, there is compelling evidence that diets need to be balanced for amino acids.
The proportion approach (or ideal protein, where we identify the proportions of amino acids in metabolizable protein supply for maximal use in milk-protein synthesis) has the advantage of being simple to use and has certainly initiated implementation of amino-acid balance in diets. The factorial approach, which estimates requirements and supply of amino acids in grams per day, is also proposed by different models, with improved accuracy over time.
- Amino acids are the “base-units” used by tissues to synthesize proteins.
- These base-units are delivered through digestion of proteins in the intestine. These proteins are a mixture of dietary, microbial and endogenous proteins, and carry a highly variable proportion of amino acids.
- Utilization of amino acids varies greatly among themselves and between tissues.
- Therefore, to maximize the match between supply and requirement, dairy rations need to be balanced on an amino-acid basis.
- Estimating and meeting requirements for individual amino acid is not yet an easy task, but first steps can be taken.