The aim of this study was to compare the concentrations of blood variables obtained simultaneously from the jugular and mammary veins of dairy cows. Eighty Holstein cows were divided into four equal groups: dry, low- (LY), medium- (MY) and high-yielding (HY). Blood insulin, glucose, non-esterified fatty acid (NEFA), beta-hydroxybutyrate (BHBA) and urea concentrations were measured. The jugular and mammary vein (J/M) ratio between concentrations of each variable was calculated. Differences between the groups of cows in concentrations of variables in the jugular vein were not in accordance with those obtained for the mammary vein. J/M values for insulin and glucose concentrations were above 1.0 in all groups of cows. The ratios for NEFA and BHBA concentrations were under or equal to 1.0 in dry and LY cows but above 1.0 in the MY and HY groups, indicating that in MY and HY cows those metabolites are apparently utilised by the mammary gland. J/M values for urea were above 1.0 in dry and LY cows but less than 1.0 in groups MY and HY, indicating that in the latter case urea is apparently released by the mammary gland. In conclusion, J/M for NEFA, BHBA and urea may be useful for estimation of the critical point when the mammary gland receives insufficient energy precursors for its current activity.
Keywords: Energy balance; dairy cow; jugular vein; mammary vein; milk yield.