@conference {41074,
	title = {Amino acid utilisation by the mammary gland: Whole blood versus plasma free amino acid pools},
	booktitle = {Proceedings of the New Zealand Society of Animal Production},
	volume = {59},
	year = {1999},
	month = {Jan},
	pages = {62-65},
	publisher = {New Zealand Society of Animal Production},
	organization = {New Zealand Society of Animal Production},
	abstract = {Utilisation of amino acids (AA) by the mammary gland of dairy cows has been studied in order to understand the responses in milk protein production to alterations in nutrient supply. One approach has been the use of balance studies, in which the amount of AA taken up from the blood by the mammary gland is compared with their respective output in milk protein(s). Although it is generally accepted that the free AA pool in blood is the major precursor for milk protein synthesis, there are differences when plasma or whole blood arterio-venous differences (A-V) are used to measure the uptake of AA by the mammary gland. Arterial and venous blood samples were obtained at 2 h intervals from cows fed fresh pasture over two 12-h sampling periods. The samples were subdivided into whole blood (WB) and plasma (P) and analysed for free AA concentrations, which were used to estimate amino acid concentrations in the erythrocyte. There were no significant differences in the A-V calculated from WB or P for the majority of AA. Plasma A-V was significantly higher than WB for threonine (13.9 vs 8.9 nmol/g blood) whilst WB A-V was higher than P for isoleucine (26.9 vs 23.3 nmol/g blood) and tyrosine (11.8 vs 10.3 nmol/g blood). Assuming the differences in A-V between WB and P can be attributed to AA exchange with the red blood cell (RBC), significant (t<0.01) exchange from RBC to the mammary gland occurred for only isoleucine (15\% of WB A-V) and tyrosine (14\% of WB A-V). These results indicate that plasma AA concentrations are valid measures of the immediate free amino acid pool available for uptake by the mammary gland. },
	issn = {1176-5283},
	author = {Pacheco-Rios D and McNabb WC and Treloar BP and Barry TN and Lee J}
}
@conference {41000,
	title = {Arterio-venous differences of amino acids across the mammary gland of cows fed fresh pasture at two levels of dry matter intake},
	booktitle = {Proceedings of the New Zealand Society of Animal Production},
	volume = {58},
	year = {1998},
	month = {Jan},
	pages = {98-101},
	publisher = {New Zealand Society of Animal Production},
	organization = {New Zealand Society of Animal Production},
	abstract = {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. },
	issn = {1176-5283},
	author = {Pacheco-Rios D and McNabb WC and Cridland S and Barry TN and Lee J}
}
@conference {40911,
	title = {The effects of methionine supply on milk composition and production of dairy cows in mid- lactation.},
	booktitle = {Proceedings of the New Zealand Society of Animal Production},
	volume = {57},
	year = {1997},
	month = {Jan},
	pages = {147-150},
	publisher = {New Zealand Society of Animal Production},
	organization = {New Zealand Society of Animal Production},
	abstract = {A trial assessing the production response of dairy cows to supplementation with methionine (Met) was conducted indoors over a 21-day period with 26 Friesian cows in mid-lactation and individually fed fresh perennial ryegrass/white clover pasture. Group 1 (n=9) received twice daily oral methionine (15 g/d) that was protected from rumen degradation (RPM; Lactet); Group 2 (n=9) was given a continuous intravenous infusion of free Met (15 g/d; MET-i.v.); and Group 3 (n=8) was given no methionine supplement (Control). Supplementary methionine tended to slightly depress feed intake but milk yield tended to be higher (P=0.08) for the MET-i.v. group compared with the Control and RPM groups (18.5 {\textpm} 0.48 vs 17.1 {\textpm} 0.45 and 17.2 {\textpm} 0.38 kg/d {\textpm} SE). Compared with the control group, there was a trend for Met (both MET-i.v. and RPM) to reduce milkfat concentration and to increase the concentration of casein. Neither lactose or whey nitrogen (N) concentrations were affected by the treatments. Methionine supplementation did not affect yields of total protein nor protein components; however, when protein and casein yields were adjusted to equal crude protein intake, Met supplemented cows produced significantly (P=0.05) more total protein and casein per unit of N ingested. These results suggest that post-ruminal Met supplementation improved the efficiency with which dietary protein was converted to casein in the pasture-fed dairy cow. More research is required to investigate which AA, individually or in combination, are likely to limit milk protein synthesis in the mammary gland of the pasture-fed dairy cow. },
	issn = {1176-5283},
	author = {Pacheco-Rios D and McNabb WC and Hill JP and Barry TN and MacKenzie DDS}
}
