Interaction of unsaturated fat or coconut oil with monensin in lactating dairy cows fed 12 times daily. II. Fatty acid flow to the omasum and milk fatty acid profile.

Feeding animal-vegetable (AV) fat or medium-chain fatty acids (FA) to dairy cows can decrease ruminal protozoal counts. However, combining moderate to large amounts of AV fat with monensin (tradename: Rumensin, R) could increase the risk for milk fat depression (MFD), whereas it is not known if diets supplemented with coconut oil (CNO; rich in medium-chain FA) with R would cause MFD. In a 6 × 6 Latin square design with a 2 × 3 factorial arrangement of treatments, 6 rumen-cannulated cows were fed diets without or with R (12 g/909 kg) and either control (no fat), 5% AV fat, or 5% CNO. Diets were balanced to have 21.5% forage neutral detergent fiber, 16.8% crude protein, and 42% nonfiber carbohydrates. Omasal flows of FA were characterized by an increased percentage of trans 18:1 for AV fat and CNO diets compared with the control, a higher percentage of 12:0 and 14:0 for CNO, and higher cis 18:1 for AV fat. Milk FA composition reflected the changes observed for omasal FA digesta flow. The de novo FA synthesis in the mammary gland was decreased by the main effects of R compared without R (averaged over fat treatments) and for added fat (AV fat and CNO) versus control (averaged over R). The percentages of 6:0, 8:0, and 10:0 in milk fat were lower for R and for AV fat and CNO compared with the control. The percentage of trans 18:1 FA in milk fat also higher for AV fat and CNO compared with the control. Against our hypotheses, the feeding of CNO did not prevent MFD, and few interactions between R and fat source were detected. The feeding of CNO did compromise ruminal biohydrogenation, with accumulation of trans 18:1 in the rumen and in milk fat.