Development of an experimental approach to measure vitamin Bn production and utilisation in sheep

摘要

Cobalt (Co) is required by ruminants to enable synthesis of vitamin B_(12) by rumen micro-organisms. Lack of quantitative information on the synthesis, absorption or utilisation of the vitamin has made determining nutrient requirement difficult. Two trials with sheep previously depleted of vitamin B_(12) and offered low Co diet (0.08 mg/kg DM) are described which attempt to overcome these problems. In the first, ewes were dosed with Co intra-ruminally at rates between 0 - 1 mg Co/day. Responses in plasma vitamin B_(12) were proportional (P < 0.001) to the amount of Co infused. In the second trial, vitamin B_(12) was infused into the bloodstream via the jugular vein at rates between 0 - 2500 nmol/day. Plasma vitamin B_(12) plateau values were directly proportional (P < 0.001) to infusion rate. The rate of vitamin B_(12) infusion needed to sustain the critical plasma concentration of 370 pmol B_(12) /1 was 5.02 nmol B_(12) /day, which together with estimated absorption from the basal diet gives a total requirement for absorption of 8.12 nmol B_(12) /day. This value is in agreement with data in the literature. The model approach, therefore, seems quantitatively robust. This approach, together with estimates of vitamin B_(12) flow from the rumen, willenable study of dietary factors that predispose to poor vitamin B_(12) production in the rumen and its absorption from the alimentary tract, and therefore to low animal vitamin B_(12) status.