Lack of production response in grazing dairy cows supplemented with long-acting injectable vitamin B12.

AIM: To determine the concentrations of vitamin B12 in serum, liver and milk that identify adequate vitamin B(12) status in grazing lactating cows, based on no change in milk production in response to supplementation with vitamin B12. METHODS: In October 2005, in early lactation, Friesian cows from one herd were injected S/C with 60 or 90 mg long-acting vitamin B12, or no injection (Control; n=39 per group, Day 0). Pasture samples were collected for Co determination at monthly intervals over 82 days. Concentrations of vitamin B12 in milk and serum (n=10 per group) and in liver (n=5 per group) were assessed over 124 days. Milk production and composition were determined on four occasions for all cows. RESULTS: Mean concentrations of Co in pasture ranged from 0.11 to 0.34 mg/kg dry matter (DM). Mean initial concentrations of vitamin B12 in milk, serum and liver were 1,520, 128 pmol/L and 1,092 nmol/kg fresh tissue, respectively. Administration of 60 and 90 mg vitamin B12 had similar effects and increased concentrations of vitamin B(12) in milk by 3-fold over controls on Day 50 (7,410 vs. 2,350 pmol/L; p<0.001) and 1.6-fold on Day 124 (3,470 vs. 2,210 pmol/L; p=0.011). Treatment with 60 and 90 mg vitamin B12 increased concentrations of vitamin B12 in serum, by at least 5-fold over controls on Day 50 (880 and 1,040 vs. 160 pmol/L; p<0.001). The two treatments increased concentrations of vitamin B12 in liver by only 1.5 fold over controls on Day 50 (1,660 and 1,900 vs. 1,200 nmol/kg fresh tissue; p<0.005). Treatment had no effect at any sampling time on daily milk volume or milk solids, fat or protein percentages. CONCLUSIONS: Concentrations of vitamin B12 in serum >128 pmol/L indicated adequate vitamin B12 status in grazing lactating cows. Supplementation with 60 or 90 mg vitamin B12 increased and maintained concentrations of vitamin B12 in serum for up to 124 days, but there was no effect on milk production. Milk was enriched in its vitamin B12 content, which would provide enhanced nutrition for calves or humans. Compared with 60 mg, the larger dose provided little additional benefit. CLINICAL RELEVANCE: Long-acting injectable vitamin B12 used to treat and prevent vitamin B12 deficiency is unlikely to improve the milk production of grazing cows when concentrations of vitamin B12 in serum are >128 pmol/L.