Lactating cows had significantly higher blood flow velocities in the milk vein than dry cows. The maximum velocity measured in group 2 (10 kg) was 38 cm/s, which was 2.7 times the velocity in dry cows. Interestingly, there was no further increase in velocity with increasing milk yield in group 3. In contrast, the blood flow volume increased linearly with increasing milk yield from 0.79 l/min in dry cows to 1.80 l/min in group 2 (10 kg; 2.3 fold) and to 3.09 l/min in group 3 (20 kg; 3.9 fold). In another study, a cow with a daily milk production of 40 kg had a blood flow volume of 4.98 l/min in the right milk vein , supporting the linear relationship between milk yield and blood flow volume in that vein. Milk yield also affected the blood flow velocity in the musculophrenic vein, but variations were large and the only significant differences were seen in the mean maximum blood flow velocity. This increased 2.7-fold from 30.59 cm/s in the dry cows to 81.90 cm/s in group 2, which was similar to the value in group 3 (85.15 cm/s). The blood flow volume also increased in the musculophrenic vein with increasing milk yield but the differences were not significant. It is surprising that milk yield had an effect on blood flow variables of a vessel not directly associated with the udder; however, lactation has shown to have an effect on the entire circulatory system . Lactating cows had a two- to three-fold higher cardiac output than dry cows, and because 20 to 30% of the cardiac output in a lactating cow is directed toward the udder, the changes are more pronounced in the milk vein than in the musculophrenic vein . The three-fold increase in the blood flow volume of the milk vein in group 3 compared with the dry cows highlights the importance of this vessel for venous drainage of the udder. A blood flow volume of 3.09 l/min translates into almost 9000 l of blood that circulate through the udder via the two milk veins per day compared with 2275 l in dry cows. Three veins are involved in drainage of the udder : the milk vein carries blood to the cranial vena cava; some blood drains toward the caudal vena cava via the external pudendal vein in the inguinal canal; and some blood drains to the caudal vena cava via the ventral perineal vein in a caudodorsal direction through the ischiadic arch and the internal pudendal vein. Which of these three routes drains the most blood has been controversial, and the exact relative contributions of the veins remain to be determined. The external pudendal vein was considered the most important in one study , whereas the milk vein was thought to be the major contributor in another . In a third study, the veins were not ranked . Estimates of the amount of blood required for the production of one litre of milk vary and have included 300 to 500 l [9, 11], 400 l , 500 l  and even up to 600 l . Assuming that approximately 500 l of blood are required for 1 l of milk, approximately 10'000 l of blood are required for 20 l of milk. Our results showed that in a cow producing 20 kg of milk, approximately 8900 l of blood flow through both milk veins in one day, which means that the milk vein is the predominant route of venous drainage of the udder, supporting findings by Kjaersgaard . Based on these calculations, approximately 90% of the venous drainage from the udder occurs via the milk veins. This confirms the well-known adage that large milk veins indicate high milk production . This should also serve as a reminder that the integrity of the milk veins must be preserved and complications such as iatrogenic thrombophlebitis must be avoided.
There were no differences between blood flow variables recorded on the left and right sides of the body. Thus, it appears that venous drainage of the udder occurs in a symmetric fashion and that in healthy cattle measurements on one side are representative of both sides. Bilateral measurements are required to objectively assess the degree of blood flow impairment in cows with a unilateral lesion. For instance, the blood flow volume in the thrombosed left milk vein of a cow was reduced to approximately one third of the blood flow volume on the right (0.79 l/min versus 2.54 l/min) .