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

Abstract

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 13 C-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.

Polyethylene glycol reduces the net flux of branched-chain amino acids across the mammary gland in ewes fed sulla

“…Higher mammary plasma fl ow and peripheral BCAA concentration could explain the elevated mammary uptake in control ewes (CT active). This agrees with results showing that BCAA are taken up by the bovine mammary gland not only for milk protein synthesis but also for oxidation or synthesis of metabolites important for mammary function (Pacheco et al, 2003).…”

“…Mammary blood fl ow was measured continuously during this period. Plasma was obtained from blood samples collected from the blood vessels mentioned previously for measuring AA (Pacheco et al, 2003) and pAH concentration (Huntington et al, 1989). One control ewe died for reasons not related to the treatment.…”

“…Uptake of BCAA by the mammary gland is usually in excess of that required for milk protein synthesis and this excess is used for other important mammary functions such as the synthesis of non-essential AA (Pacheco et al, 2003). Increased milk production and protein concentration was observed in ruminants fed fresh CTcontaining forages, suggesting that these legumes may have potential as forages for improving dairy production (Woodward et al, 1999).…”

Effects of high ambient temperature and restricted feed intake on urinary and plasma 3‐methylhistidine in lactating Holstein cows

“…High ambient temperatures are well known to reduce milk protein production in lactating dairy cows (Brash et al 2001). Nitrogen utilization for milk production by lactating dairy cows declines under high ambient temperature and feed restriction (Terada & Shioya 1998;Pacheco et al 2003). Our previous report (Kamiya et al 2005) also indicated that high ambient temperature, feed restriction, and high ambient temperature itself reduced the nitrogen utilization for milk production by lactating dairy cows.…”

“…2001). Nitrogen utilization for milk production by lactating dairy cows declines under high ambient temperature and feed restriction (Terada & Shioya 1998; Pacheco et al . 2003).…”