Effects of supplementation of dairy cattle with fish oil on silage intake, milk yield and milk composition

Summary. The eects of level of fish oil inclusion in the diet on grass silage intake, and milk yield and composition of dairy cows oered either 5 or 10 kg concentrates}d were evaluated in a ten treatment, partly balanced, changeover design experiment involving 50 cows in early lactation. Concentrates were prepared to provide 0, 150, 300 or 450 g fish oil}cow per d or 300 g fish oil}cow per d from a premix when each animal was oered 5 kg}d. The fish oil was predominantly from herring and mackerel caught in the North Atlantic while the fish oil premix was obtained from a commercial source and used palm kernel expeller as a carrier. Increasing fish oil supplementation decreased silage dry matter intake and the concentrations of milk fat and protein, and increased milk yield and diet digestibility. There were significant interactions between concentrate feed level and level of fish oil for silage intake and milk yield. Other than for the concentrations of milk fat and protein, and 20:4n‐6 fatty acids, the source of fish oil did not aect forage intake or animal performance. Fish oil supplementation also decreased the concentrations of milk protein by 0‐ 9g }kg for each 100 g increase in fish oil supplementation, the depression being similar at each level of concentrate feeding. Supplementing the feed of dairy cows with 450 g fish oil}cow per d decreased the concentration of milk fat by 15 g}kg. This study also showed that feeding dairy cattle with fish oil is an ecient method of increasing eicosapentaenoic acid in the human diet through transfer into milk. The report of the Committee on Medical Aspects of Food Policy (1994) on the nutritional aspects of cardiovascular disease recommended a reduction in the intake of both total fat and of saturated fat by the UK population. In liquid and processed forms, milk accounts for 0‐30 of total fat intake and 0‐40 of saturated fat intake (British Nutrition Foundation, 1992). Furthermore, the Committee on Medical Aspects of Food Policy (1994) recommended that the intake of long chain n‐3 polyunsaturated fatty acids including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) by the UK population should be substantially increased. Fish oils are good sources of both EPA and DHA. Milk quotas restrict the volume and butterfat concentration of milk production within the EU. As a result, there is considerable interest in developing nutritional strategies to decrease butterfat content, and consequently the fat:protein ratio, in