Volume 90, Issue 2, August (2003), pp. 271-282 © The Author 2003
doi:10.1079/BJN2003886

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Whole-body fluxes and partitioning of amino acids to the mammary gland of cows fed fresh pasture at two levels of intake during early lactation

D. Pacheco^1,2,3, M. H. Tavendale¹, G. W. Reynolds³, T. N. Barry³, J. Lee¹, W. C. McNabb^1
1Nutrition and Behaviour, AgResearch Ltd, Grasslands Research Centre, Private Bag 11008, Palmerston North, New Zealand
²Institute of Food, Nutrition and Human Health, Massey University, Private Bag 11222, Palmerston North, New Zealand
³Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Private Bag 11222, Palmerston North, New Zealand

 (Received 6 December 2001–Revised 20 February 2003–Accepted 11 March 2003)

The utilisation of essential amino acids (EAA) by the mammary gland of lactating dairy cows fed fresh forages was studied to provide basic information useful in designing strategies to increase the production of milk protein from pasture-fed dairy cows. The relationship between the flux of EAA in the whole body and their uptake by the mammary gland was determined in four cows in early lactation (length of time in milk 44 (sd 14·5) d) producing 21 (sd 4·0) kg milk/d. The cows were maintained in metabolism stalls and fed fresh perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) pasture ad libitum or restricted to 75 % ad libitum intake. The whole-body fluxes of amino acids (AA) were measured using an arterio-venous infusion of universally ¹³C-labelled AA. Whole-body fluxes of fourteen AA were estimated. Isotope dilution indicated that mammary utilisation accounted for one-third of the whole-body flux of EAA, with individual AA ranging between 17 and 35 %. Isoleucine, leucine, valine and lysine were the EAA with the greatest partitioning towards the mammary gland (up to 36 % of the whole-body flux), which could reflect a potentially limiting effect on milk protein synthesis. In the case of AA with low partitioning to the mammary gland (for example, histidine), it is suggested that non-mammary tissues may have priority over the mammary gland and therefore the supply of this AA may also limit milk protein synthesis.

Keywords:
Mammary gland, Amino acid metabolism, Dairy cows, Pasture diets

Abbreviations:
AA, amino acid, BCAA, branched-chain amino acid, DMI, DM intake, EAA, essential amino acid, GIT, gastrointestinal tract, Gln, glutamine, Glu, glutamic acid, His, histidine, IE, isotopic enrichment, Ile, isoleucine, ILR, irreversible loss rate, Leu, leucine, Lys, lysine, Met, methionine, Phe, phenylalanine, Ser, serine, TBDMS, N,O-tert-butyldimethylsilyl, Thr, threonine, Tyr, tyrosine, Val, valine