[Methodological study on measuring mammary arteriovenous differences in the dairy cow]

The effects of feeding frequency and diet composition on mammary arteriovenous (AV) differences were observed in lactating dairy cows. Two diets with 30 or 70 p. 100 concentrate (table 1) were offered 2 or 8 times daily in limited amounts. Circadian patterns of mammary AV differences in the main blood metabolites and in milk production (table 2) were studied in two cows with an exteriorized carotid. Diet digestibility and ruminal fermentation profiles were studied in two other cows fitted with a rumen cannula (tables 3, 4). 1. Circadian patterns of arterial levels and AV differences in acetate and beta-hydroxybutyrate depended on feeding frequency (fig. 2). With two feedings per day, the arterial levels reached a maximum 4 or 5 hrs after feeding, and the circadian patterns of AV differences were strongly related to those of the arterial levels. With 8 feedings per day, the circadian changes in AV differences were smaller (fig. 2), and their coefficients of variation decreased from 13 to 9 p. 100. On the other hand, packed cell volume and arterial glucose did not show any noticeable circadian changes, even with 2 feedings per day. 2. The mean arterial levels and AV differences in acetate, beta-hydroxybutyrate and most of the essential amino acids (EAA) decreased (-16, -8 and -10 p. 100) (table 5) when the feeding times were increased from 2 to 8 per day. The EAA decreased more with the high concentrate (-19 p. 100) than with the low concentrate (-8 p. 100) diet (table 6). 3. With the "high concentrate" diet, the arterial levels and AV differences in acetate were lower (-25 p. 100), while those of beta-hydroxybutyrate were higher (+13 p. 100), although the ruminal fermentations were not very different. "Continuous feeding" stabilized the AV differences, thus permitting the number of blood samples required to be reduced (50 p. 100). But this technique caused some physiological modifications, and the results obtained are not applicable to the standard practice of two feedings per day.