Metabolic fate of fatty acids involved in ruminal biohydrogenation in sheep fed concentrate or herbage with or without tannins

A 2 × 2 factorial experiment was car- ried out to evaluate the effect of herbage or concen- trate feeding and dietary tannin supplementation on fatty acid metabolism and composition in sheep rumi- nal fluid, plasma, and intramuscular fat. Twenty-eight male lambs were divided into 2 equal groups at 45 d of age and kept in individual pens. One group was given exclusively fresh herbage (vetch), and the other group was fed a concentrate-based diet. Within each treat- ment, one-half of the lambs received supplementation of quebracho powder, providing 4.0% of dietary DM as tannins. Before slaughter, blood samples were collected. The animals were slaughtered at 105 d of age, and ru- minal contents and LM were collected. Blood plasma, ruminal fluid, and LM fatty acid composition was de- termined by gas chromatography. Tannin supplementa- tion reduced (P < 0.05) the concentration of stearic acid (−49%) and increased the concentration of vacce- nic acid (+97%) in ruminal fluid from concentrate-fed lambs. Within concentrate- and herbage-based diets, tannin supplementation reduced the accumulation of SFA in blood (P < 0.05) compared with lambs fed the tannin-free diets. When tannins were included in the concentrate, the LM contained 2-fold greater concen- trations of rumenic acid compared with the LM of the lambs fed the tannin-free concentrate (0.96 vs. 0.46% of total extracted fatty acids, respectively; P < 0.05). The concentration of PUFA was greater (P < 0.05) and SFA (P < 0.01) less in the LM from lambs fed the tannin-containing diets as compared with the animals receiving the tannin-free diets. These results confirm, in vivo, that tannins reduce ruminal biohydrogenation, as previously reported in vitro. This implies that tan- nin supplementation could be a useful strategy to in- crease the rumenic acid and PUFA content and to re- duce the SFA in ruminant meats. However, the correct dietary concentration of tannins should be carefully chosen to avoid negative effects on DMI and animal performance