Mammary gland amino acid use
The mammary gland has the ability to alter the rate of its arterial blood supply and efficiency of nutrient extraction in order to obtain amino acids required for milk protein synthesis. Updated research findings have turned away from the classic limiting amino acid theory that suggests low supply of a single amino acid can limit milk protein production, and recognition is now being placed on the variable use-efficiencies of all EAA. The current method of establishing amino acids requirements as a portion of metabolisable protein does not allow for separate manipulation of individual EAA supply and results in rations with unnecessarily high crude protein contents.
Milk protein yield is particularly responsive to EAA, and when in abundant supply the mammary gland typically maintains a stoichiometric pattern of amino acid uptake. Of the EAA, methionine, histidine, tryptophan, phenylalanine, and tyrosine are taken up by the mammary gland at the same rate at which they are output into milk. These particular amino acids are often investigated as potential limiters of milk protein yield due to their tight economy of use and high mammary extraction efficiency. Lysine, threonine, and arginine, and the branched chain amino acids (leucine, isoleucine, and valine) are extracted by the mammary gland in excess of what is output in milk protein. The gland then uses this excess to synthesise non-EAA required for milk protein synthesis.
Because of the variation in fractional removal of amino acids by splanchnic and peripheral tissues, defining amino acid requirements as a fixed proportion of metabolisable protein supply is inaccurate and inefficient. The splanchnic tissues can significantly affect metabolism of dietary protein, with the largest portion of amino acids that become catabolized being those which are not extracted during their first pass through the mammary gland. Providing an optimal profile of specific EAA in the diet, in contrast to simply increasing crude protein content, could thereby improve mammary uptake, increase milk protein yield and reduce excess amino acid catabolism to minimize nitrogen excretion.
The animal on a cellular level
As established, the rate of protein synthesis at the tissue level is regulated by precursor supply, and the potential to minimise nitrogen excretion relies heavily on the mammary gland's ability to effectively use amino acids for milk protein synthesis – it is ultimately the activity of individual mammary cells that are contributing to the desired lactation performance for which we formulate and feed. It is both important and practical to measure responses at the whole-animal level, but fundamental information about how milk synthesis is controlled by nutrition on a cellular level gives scientists insight into why the mammary gland responds the way it does to varying nutritional conditions.
The mammary gland of a dairy cow is unique compared with other organs, as it undergoes rapid cell proliferation leading up to parturition and steady cell death during lactation to eventually produce dry-off – a cycle that repeats itself with each calving. Individual mammary cells are responsive to specific nutrients and hormones, and understanding how amino acids and energy can alter the cell cycle and the synthetic activity within a cell could unveil unique intervention to evoke more persistent lactation and higher yields of milk and its components. Future study should focus on harmonising the performance of the whole cow to varying profiles and amounts of metabolisable protein and the implications of diet on mammary gland cellular adaptation to influence milk protein synthesis.
Precise nutrient delivery allows maximisation of milk protein output
Insight into the details of how milk protein substrates – amino acids and energy – alter metabolic function in the dairy cow and protein synthesis at the cellular level will allow for more precise nutrient delivery and opportunities to maximise milk protein output and animal efficiency while minimising the cost and environmental footprint of the dairy industry. Inclusion of variable EAA efficiencies into ration formulation and requirement prediction models should help reduce dietary protein levels by more accurately representing amino acid partitioning in the lactating dairy cow.