Effects of altering the ratio of dietary n-6 to n-3 fatty acids on performance and inflammatory responses to a lipopolysaccharide challenge in lactating Holstein cows

The study was designed to evaluate the effects of altering the ratio between n-6 and n-3 fatty acids (FA) in the diet and the intake of these FA by lactating dairy cows on lactation performance and inflammatory acute phase responses to a challenge with lipopolysaccharide (LPS). Multiparous Holstein cows (n = 45) were blocked based on milk yield from d 6 to d 10 postpartum and, within each block, assigned randomly to 1 of 3 dietary treatments at 14d postpartum; treatments lasted for 90d. Diets were supplemented with a mixture of Ca salts of fish, safflower, and palm oils to create 3 different ratios of n-6 to n-3 FA; namely, 3.9, 4.9, or 5.9 parts of n-6 to 1 part of n-3 FA (R4, R5, and R6, respectively). During the first 5 wk of the study, blood was sampled weekly and analyzed for concentrations of metabolites and hormones. On d 75 postpartum, cows received an infusion of 10μg of LPS into one quarter of the mammary gland to evaluate inflammatory acute phase responses. Altering the ratio of dietary n-6 to n-3 FA was reflected in changes in the FA composition of plasma and milk fat. Reducing the ratio of n-6 to n-3 FA from R6 to R4 increased dry matter intake (24.7, 24.6, and 26.1 ± 0.5kg/d for R6, R5, and R4, respectively), with concurrent increases in yields of 3.5% fat-corrected milk (43.4, 45.4, and 48.0 ± 0.8kg/d), milk fat (1.53, 1.60, and 1.71 ± 0.03kg/d), milk true protein (1.24, 1.28, and 1.32 ± 0.02kg/d), and milk lactose (2.12, 2.19, and 2.29 ± 0.04kg/d). After the LPS challenge, concentrations of IL-6 in plasma increased as the ratio of n-6 to n-3 FA increased (112.5, 353.4, and 365.1 ± 86.6pg/mL for R4, R5, and R6, respectively). Elevations of body temperature and somatic cell count were greater for cows fed R5 compared with those fed R4 or R6 (41.3, 40.8, and 40.8 ± 0.2°C; 4.33, 3.68, and 3.58 ± 0.25×10⁶/mL, for R5, R4, and R6, respectively). Haptoglobin concentration was greatest at 24h after LPS challenge for cows fed R6. Phagocytosis and oxidative burst by neutrophils collected from circulation were unaffected by dietary treatment in the first 48h after intramammary LPS infusion. In conclusion, supplying the same quantity of FA in the diet of early lactation dairy cows but altering the ratio of the polyunsaturated FA of the n-6 to n-3 families influenced lactation performance and inflammatory responses to an LPS challenge.