Effects of intramuscular injections of vitamin B12 on lactation performance of dairy cows fed dietary supplements of folic acid and rumen-protected methionine

The experiment was undertaken to determine the effects of i.m. injections of vitamin B-12 on lactational performance of primiparous dairy cows fed dietary supplements of folic acid and rumen-protected methionine from 4 to 18 wk of lactation. Fourteen primiparous Holstein cows were assigned to 7 blocks of 2 cows each, according to milk production during the third week of lactation. All cows were fed a basal diet supplemented daily with rumen-protected methionine ( 18 g of supplement, to bring the estimated supply of methionine to 2.2% of metabolizable protein) plus folic acid ( 4 mg per kg of BW). Within each block, the cows received a weekly i.m. injection ( 2 mL) of saline or 10 mg of vitamin B12. Milk production was recorded daily. Milk and blood were sampled every 2 wk. Supplementary vitamin B12 increased energy-corrected milk from 25.8 to 29.0 (SE 1.6) kg/d, as well as milk yields of solids [3.52 to 3.90 (SE 0.22) kg/ d], fat [0.87 to 1.01 ( SE 0.06) kg/ d], and lactose [1.48 to 1.64 ( SE 0.11) kg/ d]. Supplementation also increased concentrations and amounts of vitamin B-12 secreted in milk but had no significant effect on dry matter intake and concentrations and amounts of folates in milk. Packed cell volume, blood hemoglobin, and serum vitamin B-12 were increased by supplementary vitamin B-12, whereas serum methylmalonic acid was decreased. Serum concentrations of sulfur amino acids were unchanged by treatment. These findings support the hypothesis that, in early lactation, supply of vitamin B-12 was not optimal and limited the lactation performance of the cows.