Quantitative aspects of blood and amino acid flows in cattle.

A quantitative literature review was undertaken on the amino acid fluxes in ruminants and the factors which influence them. Two aspects were considered: blood flow and amino acid uptake by tissues. The statistical relationships indicated that blood flow was influenced by feed intake, metabolizable energy and milk production. The rates of amino acid uptake and release by tissues and organs varied greatly between compartments and between amino acids. The net rates of appearance were the result of a combination of inter- and intratissue phenomena. Simulated balances between supply and requirements revealed the limiting role of certain amino acids, depending on milk production level and growth rate. In conclusion, this approach underlined the technical difficulties involved in obtaining this type of data, and also revealed a lack of data in certain areas. Nevertheless, the observations in this review supported the implementation of the feed units used at the present time, notably for the addition of amino acids.