Ruminal fatty acid metabolism : altering rumen biohydrolgenation to improve milk fatty acid profile of dairy cows

Nutritional guidelines promote a reduced intake of saturated fatty acids (FA) and increased intake of unsaturated FA by humans. Milk and dairy products contain a high proportion of saturated FA caused by extensive alterations of dietary lipids in the rumen through the processes of lipolysis and biohydrogenation. Therefore, marked differences exist between the FA profile in the diet (mostly unsaturated FA) and the FA profile of lipids leaving the rumen (mostly saturated FA). The objective of the research described in this thesis is therefore to improve the milk FA profile of dairy cows by altering diet composition and ruminal FA metabolism, thereby increasing ruminal outflow of unsaturated FA and consequently the secretion of unsaturated FA into milk fat. In the first study, a meta-analysis, it was shown that various fat sources, their technological form (oil, seed, protected, or addition of fish oil), and their inclusion to diets differing in forage type could significantly alter the FA profile of milk fat. In addition, the technological form of the fat source and the forage type in the basal diet affect the relationship between the dietary nutrient composition (FA and NDF content) and the milk FA profile. In the second study, various technologically and chemically treated linseed products were evaluated in vitro and it was shown that only formaldehyde treated crushed linseed and extruded whole linseed were able to decrease the extent of biohydrogenation of C18:3n3, whereas the addition of docosahexaenoic acid (DHA) to linseed oil inhibited the complete biohydrogenation to C18:0. In the third study, FA intake, omasal FA flows and plasma and milk FA profiles were measured from cows fed crushed linseed, formaldehyde treated linseed oil, extruded whole linseed, or linseed oil combined with DHA. The extent of biohydrogenation of C18:3n3 was lower for cows fed the extruded whole linseed treatment as shown by the higher omasal C18:3n3 flow compared with the other treatments. However, fat digestibility of this product was lower, resulting in no effects on plasma and milk C18:3n3 proportions. Cows fed formaldehyde treated linseed oil did show higher plasma and milk C18:3n3 proportions compared with the other treatments, but unsaturated FA content of milk fat did not differ between treatments. The cows fed linseed oil in combination with DHA showed increased omasal flows and plasma and milk fat proportions of biohydrogenation intermediates. In the final study, the milk FA profile of high producing dairy cows was evaluated after feeding an increasing proportion of crushed linseed in combination with varying forage type (grass versus maize silage) and forage to concentrate ratio. It was shown that the milk FA profile of cows fed an increasing proportion of crushed linseed depends on the forage type and forage to concentrate ratio of the diet. In conclusion, the results described in this thesis indicate that the FA profile of bovine milk fat can be altered by manipulation of the ration composition. Changes in ration composition affect ruminal FA metabolism, the profile of absorbed FA, and eventually the proportions of FA secreted in milk fat.