Mammary uptake of amino acids in response to supplementation of rumen protected lysine and methionine in early and late lactating dairy goats

Goats in early and late lactation were fed a basal diet designed to be low in lysine (Lys) and methionine (Met). The diets were either not supplemented (C), or supplemented with rumen protected Lys (L), Met (M) or both Lys and Met (LM). In order to estimate mammary supply and uptake, arterial and milk vein blood samples were withdrawn and mammary blood flow (MBF) was measured. The supplementation of both Lys and Met increased milk yield and protein production in early but not late lactation. Arterial Lys and Met concentrations were increased significantly when the amino acids were added to the feed, however, the increased concentrations did not affect mammary arterio-venous differences. The analysis of uptake in response to supply showed that the mammary gland seemed less sensitive to changes in supply in early compared to late lactation. It was concluded that it is possible to increase arterial concentration by supplementation of rumen protected Lys and Met. But supplementation to ruminants fed according to their energy and protein requirements can not be expected to have any major effects on milk production, because the mammary gland seems to possess mechanisms enabling it to adjust the extraction of Lys and Met according to its needs.