Effects of dietary n-6:n-3 fatty acid ratio on growth performance, plasma fatty acid profile, intestinal morphology, and immune function of pigs

A total of 185 crossbred pigs (Yorkshire x Landrace; 20.7 ± 2.5 d of age and 6.7 ± 1.1 kg BW) from 3 weaning groups were used to evaluate dietary n-6:n-3 fatty acid (FA) ratios on nursery and growing pigs. Within each weaning group, pigs were blocked by BW and randomly assigned to 1 of 5 dietary treatments in a randomized complete block design with balanced sex and 4 or 5 pigs/pen within each block. The dietary treatments were: 1) low n-6:n-3 FA ratio (LO): 5.0% menhaden oil (MO) and 0.0% corn oil (CO); 2) low-medium n-6:n-3 FA ratio (LO-MED): 3.5% MO and 1.5% CO; 3) medium n-6:n-3 FA ratio (MED): 1.5% MO and 3.5% CO; 4) high-medium n-6:n-3 FA ratio (HI-MED): 0.2% MO and 4.8% CO; and 5) high n-6:n-3 FA ratio (HI): 0.0% MO and 5.0% CO. The diets were fed in 2 phases as d 0 to 28 (nursery phase) and d 29 to 70 (grower phase). In vivo cell-mediated immune response was assessed on d 14 and 28 for Group 1 and 2 by measuring the inflammatory response to an intradermal injection of 100 µL of phosphate buffered saline (PBS) or phytohemagglutinin (PHA, 0.5 mg/mL of PBS). One pig was euthanized from each pen in Group 1 and 2 at the end of the nursery and grower phase to assess intestinal morphology. During the nursery or grower phase, average daily gain (ADG), average daily feed intake (ADFI), and gain to feed ratio were not affected by increasing dietary n-6:n-3 FA ratios (P > 0.16). However, pigs fed the LO diet had lower (P < 0.05) ADG and ADFI compared to those fed diets containing greater n-6:n-3 FA ratios during the nursery phase. Increasing dietary n-6:n-3 FA ratio altered plasma n-6:n-3 FA ratios on d 15 and d 29 (quadratic, P < 0.01). In vivo cell-mediated immune response to PHA did not differ among treatments. Duodenum and jejunum villus height and villus height:crypt depth ratio did not differ among pigs on different treatments at the end of either the nursery or grower phase (P > 0.10). Jejunum crypt depth increase in a quadratic manner at the end of the nursery phase (P < 0.05) but not at the end of the grower phase (P > 0.10). In summary, increasing dietary n-6:n-3 FA ratios altered plasma n-3 FA concentration and n-6:n-3 FA ratio and might influence the intestinal morphology, but did not generally affect growth performance or the skin inflammatory response of pigs in the nursery or grower phases.