Comparison of two variants of the Fick principle for estimation of mammary blood flow in dairy cows fed two levels of dry matter intake

Abstract

The estima-tion of the mammary blood flow (MBF) is an important component of studies of the utilization of metabolites by the mammary gland of lactating animals. Although there are several flow meters available to measure MBF by direct contact with the vessels perfusing the mammary gland, their use is limited mainly by the surgical preparation required for their implantation. For that reason, the application of the Fick principle was assessed as a mean to estimate MBF as part of a study on amino acid utilization by the mammary gland in three pasture-fed dairy cows at either ad libitum or restricted dry matter intakes. Two methods based on the Fick principle were assessed, namely, arterio-venous differences of amino acids (methionine: Met or phenylalanine+tyrosine: Phe + Tyr) and tritiated water (TOH). The estimated MBF was not significantly different for the Met and Phe + Tyr methods (average of 8.1 and 8.8 L min-1, respectively). The TOH method yielded a significantly lower (P < 0.05) estimate of blood flow (average of 5.3 L min-1). Using the Met and Phe+Tyr methods, the MBF was lower during the period of dietary restriction compared with the ad libitum treatment (average of 9.4 and 7.5 L min-1, respectively). In contrast, the TOH method resulted in a numerically higher MBF for the restricted group (5.7 vs. 4.9 L min-1). The short sampling period and the loss of indicator in the TOH method appear to be a disadvantage for extrapolating the estimated values to balance studies with lactating cows involving longer periods of time. The estimated values obtained using the Met or Phe + Tyr appear to be representative of the MBF during the experimental period. Therefore, any of these methods (Met, Phe + Tyr) may be used alone or in combination as an alternative to flow meters in studies of mammary metabolism.