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Optimum dietary PUFA:inhibtor concentrations for CLA enrichment in milk

Earlier work in this project demonstrated unequivocally that cis-9, trans-11 conjugated linoleic acid (CLA) in milk is derived from cis-9, trans-11 CLA formed in the rumen and also in the mammary gland via the action of stearoyl-CoA desaturase on trans-11 C18:1. Research was further extended to evaluate the potential of specific lipids in the diet to enhance milk fat cis-9, trans-11 CLA content.

Studies were conducted to examine the interaction between marine lipids in the diet that inhibit ruminal biohydrogenation and plant oils rich in C18:2 n-6 or C18:3 n-3 on the flow of fatty acids leaving the rumen and milk fatty acid composition. Fish oil (FO) was used as a source of long chain polyunsaturated fatty acids to inhibit ruminal biohydrogenation of trans-11 C18:1.

Sunflower oil (SO) and linseed oil (LO) were chosen as sources of C18:2 n-6 and C18:3 n-3, respectively, which serve as substrates for trans-11 C18:1 synthesis in the rumen. Cows were fed grass silage based diets supplemented with no oil (control), 200 g FO/day, 200 g FO and 500 g SO/day or 200 g FO and 500 g LO/day. Addition of FO in the diet decreased intake and reduced milk fat output and milk fat content, but had no effect on milk yield or milk protein secretion. Feeding FO in combination with SO or LO further decreased intake and milk fat yield, and tended to reduce milk yield and milk protein content.

Inclusion of oils in the diet induced significant changes in milk fatty acid composition, responses that were characterised as a reduction in the amount of fatty acids synthesised de novo (C6-C14), and an increase in the amount of trans C18:1, trans C18:2, total CLA, C20:5 n-3 and C22:6 n-3. Inclusion of FO in the diet increased total milk fat CLA content from 0.78 to 2.27 % fatty acids (FA). Feeding SO with FO did not increase milk fat CLA levels above those on the FO treatment (2.24 g/100 g FA), while FO and LO increased the levels of total CLA in milk yet further to 3.56 % FA.

In all cases increases in milk fat CLA content to lipid supplements were associated with a decrease in milk C18:0 content and an increase in trans 18:1 concentrations. Increases in milk trans C18:1 to lipid supplements were isomer dependent, with levels of trans-11 C18:1 being comparable when FO and FO plus SO were fed, but significantly higher for diets supplemented with FO plus LO. For diets containing FO and SO, most of the increase in trans C18:1 was due to the elevated levels of the trans-10 isomer.

The increase in milk fat cis-9, trans-11 CLA to FO was directly related to an increase in the supply of trans-11 C18:1 available for endogenous synthesis in the mammary gland and not due to an increase in preformed cis-9, trans-11 CLA. Feeding LO in combination with FO increased the flow of trans-11 C18:1 which was responsible for the further increase in milk fat cis-9, trans-11 CLA content on this dietary treatment. However, including SO to diets containing FO did not increase trans-11 C18:1 flow at the omasum above that with FO alone, but induced changes in ruminal metabolism causing trans-10 C18:1 to replace trans-11 C18:1 as the major biohydrogenation intermediate leaving the rumen.

This is the first experiment to show the importance of the relative amounts and type of polyunsaturated FA when inhibitory marine lipids are included in the diet on ruminal lipid metabolism. Inclusion of FO in the diet is effective in enhancing milk fat CLA content. Increases in milk fat cis-9, trans-11 CLA when FO is fed on grass silage based diets can be further enhanced when LO is fed, but not when SO is used as oil supplement.

This work showed that practical implementation of this nutritional approach to enhancing milk fat CLA content on-farm would need to be carefully managed to minimise the effects on intake, milk yield and milk fat content. Overall, this research has shown that when grass silage based diets contain marine lipids that inhibit the final reduction of trans C18:1 in the rumen, milk fat CLA concentrations can be further increased by linseed oil supplementation, but not when sunflower oil is fed.

Use of a biohydrogention inhibitor and oils rich in C18:3 n-3 would also be expected to be a useful nutritional strategy to enhance milk fat CLA concentrations in grazing animals at pasture as well as cows fed dried grass, ensiled grass or legume forages.

These findings are of importance in the formulation of dairy cow diets to enhance milk fat CLA content and indicate that a combination of lipids which inhibit the biohydrogenation of trans C18:1 in the rumen and are also a rich source of C18:3 n-3 is particularly effective. This research also serves to highlight the importance of ruminal lipid metabolism in the regulation of milk fat synthesis and milk fatty acid composition.

Informations connexes

Reported by

MTT Agrifood Research Finland
MTT Agrifood Research Finland
31600 Jokioinen
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