Volatile fatty acid metabolism of ruminants, with particular reference to the lactating bovine mammary gland and the composition of milk fat

Abstract

Studies are reported on the volatile fatty acid (V.F.A.) metabolism of sheep and bovines with particular reference to the association between (i) ingestion of food and ruminal levels of V.F.A.s and arterial levels of acetic acid and (ii) the utilization of arterial acetic acid by the bovine mammary gland and the association between this utilization and the proportion of lower fatty acids (Reichert-Meissl value) in the milk fat. Ruminal levels of V.F.A. and arterial levels of acetic acid were found to be similar in cattle and sheep, and similar to those reported by earlier workers for sheep. There was a close association between changes in ruminal V.F.A. and arterial acetic acid levels. Arterial acetic acid levels were found on the feeds studied to reach a maximum value of 8-14 mg. per cent. by 2-5 hours after feeding, declining to 5-8 mg. per cent. by 8 hours after feeding and to 2-6 mg. per cent. by 16 hours after feeding. On starvation for approximately 72 hours, values fell as low as 1.5 mg. per cent. Acetic acid was found to be a major metabolite of the bovine mammary gland, arterio-venous (A-V.) differences being directly dependent on the arterial level and of the order of 2-6 mg. per cent. or 40-80 per cent. of the arterial level in the fed animal. Arterial levels and mammary A-V. differences of acetic acid were unaffected by cod-liver oil feeding or low roughage-high concentrate diets, both of which depressed the fat percentage and the Reichert- Meissl (R-M.) value of the milk fat. Hyperinsulinism and recent or delayed milking also had no effect on the A-V. differences. The depression in R-M. value during fasting was not reversed by intraruminal or intravenous acetic acid infusions despite the maintenance of high blood levels of acetic acid. There was no detectable correlation between carbon dioxide output by the mammary gland and the acetic acid uptake of the gland, indicating that the acid served some 'useful' purpose in the gland. It is concluded, taking into account other evidence, that acetic acid is utilized in the gland for fat synthesis and oxidation, depending on the requirements of the gland, but that the proportion of lower fatty acids in milk fat is not dependent on the uptake of acetic acid.