Effects of dietary n-3 polyunsaturated fatty acids and ... - FINS

1 st WorkshopXIII International Feed Technology Symposiumincreased (2). This suggests that ruminal hydrogenation was more complete and thatmilk CLA synthesis was probably decreased.Feeding 4% rapeseed oil to dairy goats (35) greatly increased milk fat CLA content (by204%), and more efficiently than similar doses of rapeseed in dairy cow diets. It shouldbe stressed that the milk yield and composition responses to dietary fat differ notablybetween goats and cows. Feeding vegetable oils or seeds increases milk fat content ingoats (review by Chilliard and Bocquier (9)), whereas it generally decreases it in cows(8). This peculiarity of goats (31, 34) could be related to some differences in themetabolism of trans FA in the rumen or in the mammary gland.Feeding linseed oil (a C18:3-rich oil) greatly increases milk fat CLA content (17, 22)and is at least as efficient as C18:2-rich vegetable oils. Since C18:3 is not a precursor ofCLA in the rumen, this suggests that feeding linseed oil results in a large increase in theproduction of ruminal trans-11 C18:1, which can be used by the mammary gland forCLA synthesis.Dietary fish oil is more efficient at increasing milk CLA content than an equal amount ofplant oils. However, fish oil increases ruminal and milk (10, 11 and 12) trans-11 C18:1.So, CLA proportions increased from 0.2–0.6% with the control diet to 1.5–2.7% withdiets supplemented with fish oil (200–300 g·d–1, (14, 15).There is a linear relationship between milk fat CLA and trans C18:1 content across avariety of feeding conditions. However, the CLA: trans C18:1 ratio is much lower withfish oil.However, the effects of forage: concentrate ratio were variable for different studies, asdiscussed by Griinari and Bauman (31). Pasture feeding increases milk CLA (see above),especially with grass at an early growth stage (25). The high C18:3 content of younggrass (see above) and its low fiber content probably interact to increase the production ofCLA or its trans C18:1 precursors. Animals on pasture have a considerably higher CLAcontent in milk fat, relevant to animals fed concentrates. In fact, dairy products fromanimals on pasture may contain 300-500% more CLA compared to cattle fed a rationconsisting of 50% hay and silage and 50% concentrates. Generally speaking, fodderscontain a higher concentration of linoleic acid (C 18:3), while linolenic acid (C 18:2) ismostly contained in cereals and seeds.The influence of cow breed on milk CLA is either not significant or of limited extent,with milk from Montbeliardes showing slightly higher values (34). However, relevant tothe phase of lactation, high variability (14) of CLA content was established (9.9 to 51.7mg/g fat) in cows in the same phase of lactation, and fed the same ration.The proportions of trans-18:1 and CLA in cow’s milk produced from maize silage baseddiets (more than 60% of the ration) are small (1.1 to 2.2% and 0.4 to 0.6%, respectively)(14). CLA concentration in the milk of dairy cows switching from winter diet to young,natural meadow grass increases sharply (25, 26).Nevertheless, the milk CLA proportions measured in cows at pasture are variable (0.5 to1.7%) (Tab. 3). Milk CLA concentration increases with green grass availability (19, 25)and is further increased by lipid supplements (20). In other respects, the observedconcentrations are higher in the Spring and in the Autumn than in Summer.247