Effects of different fat sources, technological forms and characteristics of the basal diet on milk fatty acid profile in lactating dairy cows - a meta-analysis

A meta-analysis was conducted to study milk fatty acid (FA) profile in dairy cows in response to changes in dietary nutrient composition in relation to supplementation of fat sources, their technological form, addition of fish oil and main forage type in the basal diet. Data comprised 151 treatment means from 50 experiments, which were included in the database when diet composition, nutrient composition, FA composition, dry matter (DM) intake, milk yield, milk composition and milk FA profile were reported. Mixed model regression analysis including a random experiment effect and unequal variances among experiments was used. Least squares means were obtained for the different fat sources (unsupplemented, rapeseed, soybean+sunflower, linseed, or fish oil), technological form including addition of fish oil (oil, seed, protected and added fish oil), and main forage type (lucerne silage, barley silage, maize silage, grass silage, maize silage combined with haylage, or haylage) in the basal diet. Results showed that the technological form of supplemental rapeseed, soybean, sunflower, or linseed significantly influenced the effect of dietary nutrient composition on milk fat content and milk FA profile resulting in significant differences between technological forms within the different fat sources. Protected rapeseed and linseed increased C18:2n6 and C18:3n3 proportions in milk fat, respectively, whereas soybean and sunflower seed increased transfer efficiencies for C18:2n6 and C18:3n3 and their proportions in milk fat. Soybean, sunflower, or linseed supplied as oil increased trans-11-C18:1 proportions in milk fat, whereas the addition of fish oil to a diet containing soybean or sunflower decreased C18:0 and cis-9-C18:1 proportions in milk fat. The main forage type in the diet also significantly influenced the effect of dietary nutrient composition on milk fat content and milk FA profile, resulting in significant differences between main forage types in the diet within the different fat sources. Maize silage as the main forage type increased trans-11-C18:1 in unsupplemented diets or diets supplemented with a source of soybean or sunflower. For rapeseed supplemented diets, barley silage increased transfer efficiency and milk fat proportion of C18:2n6, whereas grass silage increased proportion of C18:3n3 in milk fat. For soybean or sunflower supplemented diets, haylage increased proportions of saturated FA, cis-9-C18:1 and C18:2n6, whereas the combination of maize silage and haylage increased transfer efficiency and milk fat proportion of C18:3n3. For linseed supplemented diets, grass silage as the main forage type resulted in the highest C18:3n3 proportion, whereas cis-9-C18:1 proportion was comparable for grass silage, lucerne silage and maize silage as the main forage type. This meta-analysis confirmed that the effect of dietary nutrient composition on several milk FA proportions depends on the type and form of fat supplementation, addition of fish oil, and main forage type in the basal diet.