Arterio-venous differences of amino acids across the mammary gland of cows fed fresh pasture at two levels of dry matter intake during early lactation

D. Pacheco-Rios, W.C. McNabb, S. Cridland, T.N. Barry and J. Lee

Nutrition Group, AgResearch, Grasslands Research Centre, PB 11008, Palmerston North, New Zealand.

E-Mail:pachecod@agresearch.cri.nz

Proceedings of the New Zealand Society of Animal Production. 1998, 58: 98-101

Four cows housed indoors in metabolism stalls were used in a cross-over experiment to test the effect of two levels of dry matter intake on amino acid metabolism during early lactation (October). Fresh ryegrass/white clover pasture was offered ad libitum or at 75% of the ad libitum intake during 2 experimental periods. Concentrations of essential amino acids (EAA) in arterial and mammary venous plasma were determined as indicators of the uptake and utilisation of individual AA for milk protein synthesis in the mammary gland.

The relative concentrations of free EAA (expressed as % of total EAA) of lysine, tyrosine, leucine, phenylalanine, methionine, tryptophan and histidine were lower in arterial plasma than those in the milk protein, suggesting these as limiting EAA. Using extraction rates of plasma as an indicator of potentially limiting amino acids, leucine, lysine and methionine appeared, in that order, as the most limiting amino acids in the ad libitum animals. In the intake restricted animals, the order of limitation appeared to change to methionine, lysine and leucine.

Keywords: NZSAPAB; amino acids; arterio-venous differences; milk protein; dairy cows.

Last Updated 12-09-1998