A meta-analysis of the impact of plant oils rich in 18:2n-6 on milk fatty acid 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 milk fatty acid (FA) composition in lactating cows, using meta-analysis and meta-regression approach were evaluated. Twenty experiments containing 27 comparisons between treatment and control groups, which met the selection criteria, were used. Two levels of supplementing POLA were evaluated: ≤ and > 30 g/kg diet DM. The POLA, at both level, reduced relative proportions of 6:0–16:0, enhanced 18:0, cis (Δ9–12, and 15) and trans 18:1 (Δ6−14), 18:2n-6, cis-9,trans-11 CLA, and trans-10,cis-12 CLA, whereas approximately 20 g/kg DM oil supplementation had no effect on 4:0 and 18:3n-3 concentrations in milk fat. Results of meta-regression showed that dietary 18:2n-6 content explained more than 55% of variation in results of 8:0–15:0. Dietary total FA content differences, grain level of basal diets, and parity were the sources of heterogeneity in milk cis-9 18:1. Milk trans-11 18:1 increased with increased dietary 18:2n-6 content and forage level of basal diets. Safflower oil > sunflower oil > corn oil > soybean oil > cottonseed oil induced higher standardized mean difference of milk 18:2n-6, trans-10, cis-12 CLA and 18:0 content. Feeding POLA lowered milk saturated FA and increased trans, monounsaturated and polyunsaturated FA.