The effects of fish oil supplementation on rumen metabolism and the biohydrogenation of unsaturated fatty acids in beef steers given diets containing sunflower oil

Duodenally and ruminally fistulated steers were offered grass silage and one of three concentrates at a ratio of 60 : 40 (forage: concentrate on a dry-matter basis) : F0, F1 or F4 at 14 g/kg live weight. The concentrates were designed to be iso-lipid and to provide the same amount of sunflower oil but increasing amounts of fish oil: 0, 1 and 4 g per 100 g, respectively. Ruminal characteristics were measured along with fatty acid intakes and duodenal flows to determine the effect of fish oil on : ruminal pH, ammonia-N concentration, volatile fatty acid (VFA) concentration and polyunsaturated fatty acid (PUFA) metabolism. Fish oil had no significant effect on ruminal pH, ammonia-N concentration or the molar proportions of the major VFA, although total VFA concentration was significantly reduced at the highest level of fish oil inclusion. Fish oil significantly increased the flow of long chain PUFA, total conjugated linoleic acid and vaccenic acid to the duodenum and decreased the flow of stearic acid. Biohydrogenation, as determined by the net loss of fatty acid between the mouth and duodenum, of oleic and linolenic acid was not affected by fish oil inclusion and averaged 0.64 and 0.92, respectively. There was a small increase in the biohydrogenation of linoleic acid with increasing fish oil from 0(.)89 to 0(.)92 (P < 0(.)01) on F0 and F4, respectively. Biohydrogenation of the long chain PUFA C20: 5(n-3) and C22: 6(n-3) increased from 0.49 and 0.74 to 0.79 and 0.86 (P < 0.01), respectively when fish oil in the concentrate increased from 1 to 4 g per 100 g. The net effect of fish oil on lipid metabolism appears to inhibit the transition of vaccenic acid to stearic acid in the rumen resulting in a build up of this intermediate in the biohydrogenation pathway of C18 PUFA.