Cobalt (Co) is required by ruminants to enable synthesis of vitamin B12 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 B12 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 B12 were proportional (P < 0.001) to the amount of Co infused. In the second trial, vitamin B12 was infused into the bloodstream via the jugular vein at rates between 0–2500 nmol/day. Plasma vitamin B B12 plateau values were directly proportional (P < 0.001) to infusion rate. The rate of vitamin B12 infusion needed to sustain the critical plasma concentration of 370 pmol B12 /l was 5.02 nmol B12 /day, which together with estimated absorption from the basal diet gives a total requirement for absorption of 8.12 nmol B12 /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 B12 flow from the rumen, will enable study of dietary factors that predispose to poor vitamin B12 production in the rumen and its absorption from the alimentary tract, and therefore to low animal vitamin B12 status. KEYWORDS: Co/vitamin B12; infusion; sheep; rumen; modelling.
Proceedings of the New Zealand Society of Animal Production, Volume 65, Christchurch, 19-22, 2005
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