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

AIM: To determine the concentrations of vitamin B-12 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 B-12. METHODS: In October 2005, in early lactation, Friesian cows from one herd were injected S/C with 60 or 90 mg long-acting vitamin B-12, 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 B-12 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 B-12 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 B-12 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 B-12 increased concentrations of vitamin B-12 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 B-12 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 B-12 in serum 4128 pmol/L indicated adequate vitamin B-12 status in grazing lactating cows. Supplementation with 60 or 90 mg vitamin B-12 increased and maintained concentrations of vitamin B-12 in serum for up to 124 days, but there was no effect on milk production. Milk was enriched in its vitamin B-12 content, which would provide enhanced nutrition for calves or humans. Compared with 60 mg, the larger dose provided little additional benefit.