ABSTRACT
Current knowledge of amino acid (AA) utilization by the mammary gland for milk protein synthesis in the dairy cow is based on approximately 25 years of research. Recently practical application has been derived from portions of the research dealing with transfer efficiencies for each of the essential A A (EAA). The efficiencies, which range from 1.00 to 0.42 when expressed as the ratio of output (grams per day, g/d) in milk protein to uptake (g/d) from blood by the mammary gland, are currently used in an empirical computer model (O’Connor et al., 1993) to evaluate dietary nutrient availability vs. requirement. The differ ence between uptake and output of an EAA represents catabolism within the gland to form nonessential AA or other metabolic intermediates used to support milk synthesis. Metabolic fate of AA at the cellular level is described in considerable detail by a mechanistic computer model (Baldwin et al., 1994) that was designed primarily for planning and evaluating basic research. Although these models presently may be a simplification of complex interactions within the lactating cow, they have the potential to consolidate and improve our understanding of A A utilization for the purpose of reducing nitrogen (N) loss in urine. A primary objective of current research is to reduce excretion of absorbed AA N by reducing catabolism of AA in the mammary gland and other tissues.
