Investigations on the effect of a niacin supplementation to three diets differing in forage-to-concentrate ratio on several blood and milk variables of dairy cows

The objective of this study was to investigate the influence of a niacin supplementation to three diets with different forage-to-concentrate ratios on blood and milk parameters. Seven midlactation (102 18 days in milk) and three dry cows of the Holstein-Friesian breed, equipped with cannulas in the dorsal sac of the rumen and proximal duodenum, were used. On a dry matter basis the rations applied consisted of either 1/3 concentrate and 2/3 forage (LC), 1/2 concentrate and 1/2 forage (MC), or 2/3 concentrate and 1/3 forage (HC). They were fed for one period without and in the following with a supplementation of 6 g niacin (nicotinic acid) per cow per day. The basal niacin content was 35.0 mg/kg for LC, 34.6 mg/kg for MC and 34.4 mg/kg for the HC diets on a dry matter basis. Blood was sampled before, then three and six hours after first morning feeding from the vena jugularis externa. Milk samples were obtained on two days. Niacin feeding enhanced concentrations of glucose and urea in serum (p < 0.05). No effect of supplemental niacin was seen on serum non-esterified fatty acids or beta-hydroxybutyrate concentrations. Increasing the proportion of concentrate and niacin supplementation enhanced serum nicotinamide concentration (p < 0.01). Nicotinamide concentrations or amounts excreted via milk were influenced by forage-to-concentrate ratio (p < 0.05), but not by niacin supplementation. Milk fat composition showed an effect of additional niacin, as the proportion of oleic acid increased in milk of niacin supplemented cows (p = 0.04). For all parameters analysed, no significant interaction between forage-to-concentrate ratio and niacin supplementation was found