Nutritional control of amino acid supply to the mammary gland during lactation in the pig

Much of the progress in understanding the physiology of lactation and the control of nutrient supply to the mammary gland has been achieved in the dairy cow.This is partly due to the combined importance of efficient milk production and its use as a major dietary component for man.Another reason is the relative ease of using the bovine model compared with the pig with regard to handling and milking.The availability of equipment that permits accurate, easy and direct measurement of total milk output in the cow certainly facilitates lactational metabolism studies in that species.Unfortunately, it is still difficult to accurately estimate milk output in the sow.Consequently, there is a dearth of information relating physiological and metabolic mechanisms to lactational performance as a basis for advancing the study of sow nutrition.It is increasingly apparent that refinements in nutritional strategies are not keeping pace with production improvements made by geneticists.Free amino acids in blood constitute the major precursors of milk proteins, whether provided from the dietary or body protein pool, although the mechanism of delivery of these amino acids to the milk-protein synthetic machinery remains a puzzling and challenging question.Recent advances in amino acid nutrition of the lactating sow suggest that an optimum dietary ratio of the essential amino acid valine relative to lysine maximizes milk production in sows nursing large litters (Pettigrew, 1993;Richert et al. 1996).While the availability of these amino acids in the blood to the mammary system is critical for optimizing milk production, factors yet to be studied, such as blood flow and regulation of amino acid transporter systems across mammary cells, may also influence the rate of uptake.The present paper will discuss factors involved in protein metabolism as they pertain to amino acid nutrition, and a new approach to dietary control of amino acid supply for milk synthesis.It is suggested that a better understanding of whole-body protein metabolism and the control of amino acid availability to, and utilization by, the mammary gland would provide an improved basis for estimation of amino acid requirements of the lactating sow.