Date Published: March 13, 2012
Publisher: BioMed Central
Author(s): Ueli Braun, Eva Forster.
This study investigated the effect of milk yield on blood flow variables in the milk vein and musculophrenic vein in dairy cows.
Five healthy dry cows, five cows with a daily milk yield of 10 kg and five others with a daily milk yield of 20 kg underwent B-mode and colour Doppler sonographic examination. The diameter of the veins, blood flow velocities and blood flow volumes were measured on both sides in standing, non-sedated cows using a 7.5 MHz linear transducer.
Lactating cows had significantly higher blood flow velocities in the milk vein than dry cows; the maximum blood flow velocity of dry cows and those with a daily milk yield of 10 and 20 kg were 14.04, 38.77 and 39.49 cm/s, respectively, the minimum velocities were 0.63, 3.02 and 2.64 cm/s, respectively, and the mean maximum velocities were 8.21, 26.67 und 28.22 cm/s, respectively. Cows producing 20 kg of milk a day had a blood flow volume of 3.09 l/min, which was significantly higher than 0.79 l/min recorded in dry cows. Lactating cows had significantly higher mean maximum blood flow velocities in the musculophrenic vein than dry cows. Blood flow variables of both veins did not differ significantly between the left and right side.
This study showed that milk yield has a profound effect on blood flow variables in the milk vein and to a lesser extent the musculophrenic vein. This must be taken into consideration in future Doppler sonographic studies of these veins and possibly other vessels. Furthermore, measurements on one side are representative of both sides.
The ultrasonographic characteristics of various arteries and veins in cattle have recently been described using colour Doppler [1-3]. The milk vein and musculophrenic vein are particularly amenable to Doppler ultrasonographic examination because of their superficial location, and cows tolerate examination of these veins better than the jugular vein . The milk vein drains blood from the udder. However, it is not known whether Doppler sonographic characteristics of blood flow vary with the amount of milk produced or whether the stage of lactation needs to be considered when interpreting results. The present study tested the hypothesis that milk yield has an effect on Doppler sonographic findings of the milk vein but not of the musculophrenic vein because the latter drains blood from the costal parts of the diaphragm and from the wall of the thorax and abdomen . To achieve this, five dry cows, five cows that produced 10 kg of milk per day and five others that produced 20 kg of milk per day underwent Doppler sonographic examination of the milk vein and musculophrenic vein. A secondary goal was to determine whether blood flow variables in these two veins differed between the left and right sides, as has been reported in goats . The findings of this study would be useful for the interpretation of previously established normal ranges of blood flow variables of the milk vein and musculophrenic vein [1-3] in relation to milk yield.
The study protocol was approved by the Animal Care Committe of the Canton of Zurich, Switzerland.
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.
This study showed that milk production has a profound effect on blood flow variables of the milk vein and to a lesser extent the musculophrenic vein. This must be taken into consideration in future Doppler sonographic studies of these veins and possibly other vessels. Furthermore, measurements on one side are representative of both sides. In cows with unilateral thrombophlebitis of the milk vein, bilateral measurements serve to estimate the degree of impairment on the affected side.
The authors declare that they have no competing interests.
UB initiated and planned the study and he prepared the manuscript. EF carried out the ultrasonographic examinations under supervision of UB. Both authors have read and approved the manuscript.
Department of Farm Animals, University of Zurich.