The effect of varying the amount of linseed oil supplementation on rumen metabolism in sheep

The effects of three levels of linseed oil (LSO) supplementation of a basal diet on rumen digestion and flow of nutrients to the proximal duodenum of three mature sheep provided with permanent rumen and duodenal re-entrant cannulas were studied. 2. A basal diet of 200 g hay and 400 g concentrates daily, providing approximately 7.0 MJ digestible energy and 13 g N/d, was given alone or with supplements of 13, 26, or 40 ml LSO/d in two equal portions of 06.00 and 18.00 hours. The flow of duodenal digesta was measured by spot-sampling using chromic oxide paper as the marker. Bacterial protein synthesis (BPS) was measured by the diaminopimelic acid technique. 3. Addition of LSO reduced the digestion of energy and organic matter, particularly acid-detergent fibre, in the stomach. Digestion in the intestines increased but at the higher levels of supplementation this failed to compensate completely for the reduction in rumen digestion. Total volatile fatty acid concentrations were not affected but molar proportions of acetate and butyrate were decreased by approximately 18 and 61% respectively while the molar proportion of propionate was increased twofold by the highest concentration of oil. The higher concentrations of LSO virtually eliminated protozoa from the rumen. 4. The second increment of LSO (26 ml/d) produced the highest duodenal flow of total N and bacterial N and the highest efficiency of BPS. The highest concentration of oil (40 ml/d) was without effect. Rumen and duodenal ammonia concentrations and plasma urea concentrations tended to be reduced by the higher concentrations of LSO. 5. It is argued that the results support suggestions made elsewhere that free oils reduce the efficiency of BPS but that they also reduce the numbers of protozoa which can cause an increase in efficiency of BPS. The net effect of free oil supplementation on BPS is thus likely to be variable and difficult to predict.