Utilisation of fish oil in ruminants: I. Fish oil metabolism in sheep

Five sheep with rumen and abomasal cannulae were offered three diets sequentially in the order: control (C) pellets (lucerne hay-oat grain: 60/40, w/w), control plus unprotected tuna oil (UTO pellets), and control plus tuna oil protected (casein-formaldehyde matrix) against ruminal biohydrogenation (PTO pellets). In supplemented diets, tuna oil constituted 3% (w/w) of total dry matter (DM), and each supplement was fed for 12 days, with 9 days allowed between the two fish oil feeding periods to minimise carry-over effects. Daily DM intake was 785±38 g/head during the control period. It was significantly reduced by UTO feeding (6.2%, P<0.05) but not PTO feeding. The level of EPA in the abomasum during PTO feeding was double that measured during UTO feeding (1.30 versus 0.61% of FA, P<0.05). The level of DHA in the abomasum did not significantly differ between UTO and PTO feeding periods. Both tuna oil supplements significantly increased the levels of 18:1 trans and that of a fatty acid derivative identified as 10-hydroxystearic acid (10-HSA) in both the rumen and abomsum. Tuna oil supplementation also altered the fatty acid composition of plasma lipid fractions and 10-HSA was solely incorporated into plasma free fatty acids. This study indicates that substantial protection of tuna oil against ruminal hydrogenation inhibited reduced feed intake, but increased the proportion of 18:1 trans isomer and fatty acids derivatives (10-HSA), which indicate interference with metabolism in the rumen.