Influence of methionine supply on the response of lactational performance of dairy cows to supplementary folic acid and vitamin B12

The present experiment was undertaken to determine if the effects of supplementary folic acid on lactational performance were caused by improved methylneogenesis and if the supply in vitamin B-12 could affect this metabolic pathway. In this eventuality, supplementary Met, a major source of preformed methyl groups, should reduce the requirements for these vitamins. Sixty multiparous Holstein cows were assigned to 10 blocks of 6 cows each according to their previous milk production. Within each block, 3 cows were fed a diet estimated to supply Met as 1.83% metabolizable protein and 3 cows were fed the same diet supplemented with 18 g of rumen-protected methionine (RPM) to supply Met as 2.23% of metabolizable protein. Within each level of Met, cows received no vitamin supplement or weekly intramuscular injections of 160 mg of folic acid alone or combined with 10 mg of vitamin B-12 from 3 wk before to 16 wk after calving. There was no treatment effect on dry matter intake during pre- and postcalving periods: 13.4 +/- 0.4 and 21.8 +/- 0.4 kg/d, respectively. Milk production was not affected by RPM supplementation. Folic acid and vitamin B-12 given together tended to increase milk production during the 16 wk of lactation. This effect was more pronounced during the first 4 wk of lactation: 37.5, 37.7, and 40.3 +/- 0.9 kg/d for cows receiving no vitamin supplement, folic acid alone, or folic acid combined with vitamin B-12, respectively. Milk fat yield was not affected by treatments. Lactose, crude protein, and total solid yields were greater, in early lactation, in cows injected with folic acid and vitamin B-12 together but this effect diminished as lactation progressed. Intramuscular injections of folic acid alone or combined with vitamin B-12 tended to decrease plasma concentrations of homocysteine from 5.51 mu M with no vitamin supplement to 4.54 and 4.77 +/- 0.37 mu M, respectively. Results of the present experiment suggest that the effects of the combined supplement of folic acid and vitamin B-12 on lactational performance of dairy cows were not due to an improvement in methyl groups supply, because RPM supplement, a source of preformed methyl groups, did not alter the cow responsiveness to vitamin supplements.