External pudic venous reflux into the mammary vein in lactating dairy cows.

A representative blood sample from the mammary vein depends on the functional integrity of the valves in the external pudic vein (EPV). To determine if the EPV valves maintain blood flow into the inguinal direction during the second and subsequent lactations, we used eight lactating cows catheterized in the EPV, the lateral branch of the cranial mammary vein (MV), and the external pudic artery (EPA). The averaged daily milk yields were 25.0 +/- 1.8 kg in cows in second lactation and 31.5 +/- 2.9 kg in older cows. The relative time taken by a pulse dose of p-amino hippuric acid (PAH) injected into the EPV, to reach the EPA and the MV, was measured in a first trial. In a second trial, we assessed the extent of alteration of the mammary PAH blood concentration with blood originating from other tissues using a continuous infusion of PAH into the EPA simultaneously with blocking or not any EPV backflux. From the first experiment, the PAH injected into the EPV appeared first in the EPA and then in the MV in cows in second lactation, suggesting that blood flow was towards the inguinal region. But in a third-lactation cow, the order of appearance was reversed. In parallel, the occlusion trial demonstrated that the concentration of PAH in the MV was diluted by 14 to 39% with blood draining nonmammary tissues only in cows in third or fourth lactation. This resulting reversed flow from the EPV towards the MV would have a detrimental impact on conclusions of mammary gland metabolism studies conducted with cows in their third lactation or higher.