The effect of a Niacin supplementation to different diets on ruminal fermentation and flow of nutrients to the duodenum 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 (F:C ratio) on ruminal metabolism of dairy cows. The rations consisted of either 2/3 forage and 1/3 concentrate, 1/2 forage and 1/2 concentrate or 1/3 forage and 2/3 concentrate on dry matter basis. Each diet was fed in one period without and in the following period with a supplementation of 6 g niacin (nicotinic acid, NA) per cow and day. Three dry and seven mid-lactating Holstein-Friesian cows, equipped with cannulas in the dorsal sac of the rumen and proximal duodenum were used. Ruminal fluid was obtained before and six times after the morning feeding, while duodenal chyme was collected every two hours for five consecutive days. Cr 2 O 3 was used as flow marker. NA supplementation increased rumen ammonia concentration, whereas it decreased short-chain fatty acid concentration. The amount of organic matter reaching the duodenum was enhanced if niacin was added to the rations. NA supplementation also led to higher flows of microbial protein and undegraded feed protein to the duodenum. Furthermore, the efficiency of microbial protein synthesis was enhanced in supplemented animals. The amounts of total niacin (the sum of NA and NAM) reaching the duodenum rose with increasing concentrate proportion and also with NA supplementation, whereas amounts of nicotinamide were only influenced by NA fee ding and not by the F:C ratio.