Interaction of sunflower oil with monensin on milk composition, milk fatty acid profile, digestion, and ruminal fermentation in dairy cows

• Sunflower oil increases proportions of total trans fatty acids in milk fat.
• Monensin at 16 mg/kg diet has no effect on total trans fatty acids of milk fat.
• There is no interaction between monensin and sunflower oil for milk fatty acid profile.

Four ruminally fistulated multiparous Holstein cows were assigned to a 4 × 4 Latin square design to determine the interaction between sunflower oil and monensin supplementation on intake, milk production, total apparent digestibility of the diet (TTAD), ruminal fermentation characteristics and milk fatty acid profile. The experimental diets consisted of a 2 × 2 factorial arrangement of treatments: (1) control (no sunflower oil and no monensin; CON), (2) diet containing (dry matter (DM) basis) 42 g/kg sunflower oil (OIL), (3) control with monensin (16 mg/kg of DM; MON), and (4) diet containing (DM basis) 42 g/kg sunflower oil and 16 mg/kg monensin (MIX). Sunflower oil supplementation tended to decrease DM intake and yield of milk fat. Sunflower oil decreased milk urea N concentration likely as a result of better N utilization, suggesting that sunflower oil contributes to decrease deamination and amount of amino acids used for gluconeogenesis. There were interactions between monensin and oil supplementation for acetate, propionate, and the acetate to propionate ratio in the rumen as a result of lower proportion of acetate and higher proportion of propionate for cows fed MON compared to those fed CON. Molar proportions of butyrate and isobutyrate in the rumen were decreased by monensin and proportion of butyrate was increased by oil. There was no interaction between monensin and oil for milk FA profile. Sunflower oil supplementation decreased proportions of all short-chain and most medium-chain FA. Compared with cows fed no oil, those supplemented with sunflower oil had higher proportions of total trans FA and monensin had no effect. This study suggests that supplementing dairy cow diets containing a corn-based concentrate with a combination of monensin and sunflower oil adds no further improvements in milk FA profile to supplementing sunflower oil alone although the lack of effect on parameters with large numerical changes such as DM intake and milk yield should be confirmed using more animals.