To investigate the effects of niacin supplementation in the diet of high producing cows at early lactation, 21 holstein dairy cows were used in this experiment. Animal were assigned in to three groups based on their milk yield and calving date soon after parturition. They were received a basal diet and 0 (group 1), 6 (group 2), 12 (group 3) g of supplementation niacin per day over a 10 weeks experimental period. Milk volume was recorded and milk samples were collected for each cow at two weeks interval for analysis of fat, protein, lactose and SNF (Solid-None Fat). Blood samples were also taken for the measurement of glucose, triglyceride, Beta-hydroxy butyrate and total protein at two weeks intervals. No significant difference were observed between milk yield, milk fat , protein, lactose and SNF content in cows received niacin compared to the control group (p>0.05). Plasma glucose in groups 2 and 3 compared to the control were higher and this difference were statistically significant (p<0.05). Blood triglycerides were not significantly affected by niacin supplementation. BHBA were lower in cows received niacin and this difference were significant (p<0.05). The trend of changes in the amount of blood total protein were identical in all three groups whole the level of this factor was always higher in control group compared to the others groups. Niacin has showed an increase in the level of plasma glucose and a notable decrease in the amount of blood triglyceride, β -hydroxy butyrate and total protein, which may be due to the effect of this vitamin on the energy metabolism in cows.

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