Nutritional and physiological aspects of supplementing niacin and dietary fat in lactation rations

Typescript (photocopy).Four studies were conducted to evaluate effects of supplemental niacin and dietary fat in lactation rations. In Experiment 1, 28 lactating Holstein cows were used to compare effects of niacin and whole cottonseed (WCS) on intake, milk yield and composition, and systemic responses. In Experiment 2, four non-lactating Holstein heifers with ruminal and duodenal cannulae were used to determine the influence of niacin and WCS on rumen fermentation, protein degradability, and nutrient digestibility. Treatments were identical to those in Experiment 1, consisting of concentrate, corn silage, chopped hay, and 0 or 6 g niacin per head daily with 0 or 15% WCS in the diet. In Experiment 3, comparative effects of four levels of WCS, niacin, and niacinamide on in vitro fermentation were determined with a continuous culture system. In Experiment 4, 28 lactating Holstein cows were used to evaluate effects of four levels of supplemental niacin on feed intake, milk yield and milk composition. In Experiment 1, both niacin and WCS increased milk fat percentage. Milk protein percentage and yield were greatest with supplementation of niacin, but were lower with cottonseed feeding. The milk protein depression with WCS was avoided by niacin, due to simulation of casein synthesis. Supplemental niacin tended to elevate plasma glucose and insulin, while WCS tended to reduce these. Plasma urea nitrogen was higher in cows fed WCS. Free tryptophan in plasma tended to be greater in cows fed niacin. In Experiment 2, niacin increased rumen protozoa and microbial protein synthesis compared to WCS. Ruminal degradation of feed protein was greatest with niacin. Digestibilities of crude protein and neutral detergent fiber were higher with niacin supplementation. In Experiment 3, increasing WCS tended to increase fermenter acetate production. Microbial protein synthesis was reduced with 30% WCS. Supplementation of either niacin or niacinamide stimulated microbial protein production compared to the control. Shifts in volatile fatty acids were similar among treatments. In Experiment 4, the level of niacin did not affect feed intake or milk yield. However, milk fat tended to be reduced and milk protein increased by adding niacin to diets containing WCS