Technical note: Comparison of Doppler ultrasound of the milk vein and the Fick principle to estimate mammary plasma flow on the day of calving and at 7 days in milk in multiparous Holstein cows

The aim of this study was to describe the utility of Doppler ultrasonography of the milk vein (MV) to assess mammary plasma flow (MPF) and determine whether a relationship exists between MPF assessed via Doppler ultrasound of the MV (MPF MV ) and the Fick principle (MPF Fick ) on the day of calving and at 7 DIM. A total of 11 parity 2 (P2) and 10 parity 3 (P3) Holstein cows were enrolled in the study immediately after calving. The MPF Fick was estimated using the mammary arteriovenous difference of Phe and Tyr as internal markers at 2, 4, 6 and 8 h after colostrum collection and the morning milking at 7 ± 1 DIM. Milk was collected at milking 2 and the evening milking at 7 ± 1 DIM for determination of milk protein output. The MPF MV was also estimated by Doppler ultrasound of the left and right MV at 2 and 8 h after colostrum collection and the morning milking at 7 ± 1 DIM. Both MPF MV and MPF Fick increased by 15.8 and 19.5% from calving to 7 DIM, respectively. No differences were detected between parity in MPF Fick ; however, P3 cows had greater MPF MV than P2 cows, as well as greater MV diameter. No relationship existed between MPF MV and MPF Fick during the calving and fresh periods, which may have been due to the violation of multiple assumptions of the Fick principle as a result of the fluctuating rate of milk secretion, tissue metabolism, and blood flow associated with the onset of lactation. Although Doppler ultrasound of the MV may be a time-efficient, non-invasive technique to estimate fresh period MPF, further work is needed to validate its accuracy with techniques estimating MPF directly in the pudendal artery, such as the dye dilution technique. Such knowledge would allow for the widespread utilization of this non-invasive technique to uncover knowledge gaps pertaining to the relationship between fresh period mammary metabolism and milk production.