Vitamin B12 administration for milk fat synthesis in lactating dairy cows fed a low fiber diet.

Thirty lactating Holstein cows were in two groups in a study of effects of vitamin B12 injections on milk fat synthesis. All cows were fed a normal fiber diet for the first 28 days after calving and then adjusted gradually to a low fiber diet over the next 28 days. After adjustment to the low fiber diet, in a single reversal trail, cows received either 150 mg of vitamin B12 in the form of hydroxocobalamin intramuscularly every 7 days for 21 days or no treatment for 21 days. Daily milk yield (kg), percent milk fat, and milk fat yield (g) for the normal fiber, low fiber adjustment, low fiber control, and low fiber plus vitamin B12 treatments were 29.6, 3.59, 1192; 31.5, 2.85, 840; 28.0, 2.58, 715; and 28.8, 2.65, 760. Injections of vitamin B12 did not correct the milk fat depression associated with the low fiber diets. In addition, there was no consistent relationship between blood B12 and milk fat production. Milk fat production was highly correlated with molar percent acetate in the rumen .63 and with blood acetate concentration .74.