Effect of Ratio of Roughage to Concentrate and Level of Feed Intake on Ovine Ruminal Vitamin B12 Production

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Abstract
Three sheep, fitted with intestinal reentrant and rumen cannulas, were used in two experiments to estimate ruminal production of vitamin B12 and to assess the effects of proportion of concentrate in the diet and level of feed intake. The flow of vitamin B12 through the duodenum over 24-hour periods was used as an estimate of daily production. Two vitamin B12 assays were used, one of which (Ochromonas malhamensis) is quite specific for the vitamin, the other of which (radioisotope dilution) responds to at least some of the analogues of vitamin B12 as well. The primary ingredients of the diets, which were ground and pelleted, were timothy hay and corn in ratios of 100:0, 70:30 and 40:60. Cobalt (0.5 ppm) was added to all diets. Average estimates of daily vitamin B12 production ranged from 579 to 1195 µg. The high corn diet significantly (P < 0.05) reduced vitamin B12 production. The production of analogues of B12 was not, however, proportionately reduced. Consequently a smaller percentage of the intestinal flow of total vitamin B12 activity was represented by the vitamin itself. This was reflected in a similar pattern in serum B12 activity. Within the range of intakes studied (approx. one to two times maintenance), vitamin B12 production responded in a linear fashion to changes in digestible dry matter intake. Vitamin B12 represented a significantly (P < 0.05) increasing proportion of total B12 activity as feed intake increased. The increased synthesis of vitamin B12 appeared to be at the expense of analogues.