Technical Note: Can tail arterial or tail venous blood represent external pudic arterial blood to measure amino acid uptake by mammary gland of cows?

• Arteriovenous difference approach is widely applied for estimating the uptake of nutrients.
• Sampling artery blood is hard in practice.
• Validate tail blood sample can be used for measuring the uptake of amino acids by the mammary gland.

Sampling blood from the artery supply and venous drainage of the udder of dairy cows is essential for estimating the uptake of amino acids (AA) by the mammary gland using the arteriovenous difference approach. Since it is difficult to sample blood form the external pudic artery, finding a representative alternative was necessary. In this experiment, 13 lactating Holstein dairy cows were used to validate whether blood from the tail artery or tail vein could be used a substitute for external pudic arterial blood for calculation of amino acid (AA) extraction rate by the mammary gland. The results showed that no significant differences were noted in the individual AA concentration (P>0.10) between the tail arterial, tail venous blood and external pudic artery, except for Asp, Thr and Pro. The calculated extraction rates based on the A-V difference between the external pudic artery, tail artery or tail vein and the subcutaneous vein were not different for individual AA (P>0.10) except for those rates for Asp, Thr and Glu (P<0.10). These results indicated that blood sample from the tail artery or tail vein could be used as an alternative to external pubic arterial blood for studying the uptake of most AA, but not for Asp, Thr and Glu, by the mammary gland of dairy cows.