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The predominant fatty acids in milk are the long-chain fatty acids myristic, palmitic and stearic. These saturated fatty acids account for 75% of the total fatty acids, with a further 21% occurring as monounsaturated fatty acids of which the most prevalent is oleic acid. Only 4 g/100 g of the milk fatty acids are polyunsaturated, occurring mainly as linoleic and linolenic acids. All milk fatty acids are derived, almost equally, from either de novo synthesis or directly from preformed fatty acids in the diet. There are four main dietary sources of fatty acids: forages, oilseeds, fish oil and fat supplements. The digestive tract exerts a profound influence on the fate of dietary fatty acids. The short-chain saturated free fatty acids are absorbed through the walls of the rumen or abomasum into the bloodstream. The medium- and longer-chain saturated fatty acids pass into the small intestine, diffuse across the membrane wall where they are incorporated into lipoproteins and enter the bloodstream via the lymphatic system. The majority of unsaturated fatty acids are extensively hydrogenated in the rumen. However, recent work has shown that the levels of certain saturated fatty acids can be reduced and the levels of oleic, linoleic and linolenic fatty acids increased by feeding oilseeds rich in mono- or polyunsaturated fatty acids. In addition, work reported here has confirmed that eicosapentaenoic and docosahexaenoic acids can be transferred to milk when a diet containing fish oil is fed, but the transfer efficiencies are low.

Dietary polyunsaturated fatty acid

The aim of this work was to test the robustness of the 0.68 estimate of the efficiency of conversion of metabolisable protein into true milk protein (Agriculture and Food Research Council (AFRC), 1993) for protein-limiting diets and to determine whether a different value is appropriate for practical rationing. Seventy-two multiparous cows were blocked on the basis of milk energy output per unit of dry matter intake (DMI), and allocated at random to one of four treatments. Treatments supplied metabolisable energy (ME) at a fixed level to individuals within a block, but varied metabolisable protein (MP) supply from 25% below the estimated requirements, through 212.5% and 112.5% up to 25% above requirements for the average performance of animals within blocks at the start of the study. Cows were offered diets to meet their predicted ME requirements for each 3-week period with measurements performed in the last week of each period. Milk protein output was regressed against the estimated MP available for production for each cow and the efficiency of conversion of MP into milk true protein was calculated, assuming a maintenance requirement according to the MP system. The efficiency of conversion of MP into milk true protein decreased with the increasing supply of MP from 0.77 to 0.50. Using an iterative approach to determine the best fit of the data when supply matched requirement resulted in a range of efficiency values between 0.62 and 0.64 g of true milk protein per g of MP.

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No abstract

The Committee on Medical Aspects of Food Policy report Nutritional Aspects of Cardiovascular Disease recommended increased dietary intakes of the long chain polyunsaturated fatty acids eicosapentaenoic acid and docosahexaenoic acid to help protect against cardiovascular disease. There is some evidence that a-linolenic acid can be elongated and desaturated by mammalian enzyme systems to produce small amounts of eicosapentaenoic and docosahexaenoic acids. Whole linseeds are rich in a–linolenic acid, and it has been suggested that feeding this oilseed to dairy cows may result in higher levels of the long chain polyunsaturated fatty acids in bovine milk. To examine this hypothesis, the effect of feeding whole ground linseed and xylose treated whole ground linseed on the performance and milk composition of dairy cows was investigated.Either form of whole linseed reduced dry matter intakes and milk protein concentration. The xylose treated whole linseed increased milk yield, but neither linseed source changed milk fat concentration. Levels of milk a-linolenic acid increased to 1.2 g/100 g fatty acid when whole ground linseed was fed, and to 1.5 g/100 g fatty acid with the xylose treated linseed compared with 0.7 g/100 g fatty acid for cows fed the control diet. The efficiency of incorporation of additional dietary a-linolenic acid into milk fat was estimated to be 1.6 and 3.4% for the whole ground and xylose treated whole ground linseed respectively. Feeding whole ground linseed increased the proportion of a-linolenic acid in milk fat, but there was no evidence of eicosapentaenoic and docosahexaenoic acids in milk fat, suggesting that if any elongation and desaturation of a-linolenic acid occurs in dairy cows, none is transferred to milk.

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