A meta-analysis of the impact of plant oils rich in 18:2n-6 on digestibility, milk production and composition in lactating cows

Including plant oils rich in unsaturated fatty acids in lactating cow diets can be used as a nutritional strategy to alter milk fatty acid composition for long-term human health. Effects of dietary plant oils rich in 18:2n-6 (POLA) on digestion, milk production and composition in lactating cows, using meta-analysis and meta-regression approach were evaluated. Twenty-five experiments containing 38 comparisons between treatment (with plant oils) and control (without plant oils) group, which met the selection criteria, were used. Two levels of supplementing POLA were evaluated: ≤ and > 30 g/kg diet DM. Overall, plant oils decreased dry matter intake (DMI) and increased milk production and the weighted mean differences were –0.30 and 0.32 kg/d, respectively, but the results were heterogeneous. Forage type of basal diet was the source of heterogeneity for standardized mean difference (SMD) of DMI, whereas dietary 18:2n-6 content differences between treatment and control diets explained the variation in results of milk yield. Feeding POLA up to 30 g/kg diet DM improved organic matter and fiber total-tract digestibility. Milk fat content and secretion lowered at both level of POLA inclusion, but milk protein and lactose output did not change. Amount of plant oil in the diet and milk yield differences between groups showed negative and positive influences on SMD of milk fat content and yield, respectively. Feeding POLA improved milk production efficiency with decreased DMI and increased milk production.