Kinetics of niacin supplements in lactating dairy cows.

The kinetics of niacin supplements in lactating dairy cows and the stability of supplements during in vitro fermentation were examined. Four multiparous Holstein cows (200 DIM) with ruminal and duodenal cannulas were fed a TMR either unsupplemented or supplemented with 12 g/d of nicotinic acid, 12 g/d of nicotinamide, or 6 g/d combination of each niacin source in a 4 x 4 Latin square design. Ruminal and duodenal concentrations of nicotinic acid increased with niacin supplementation, but DMI, yields of milk and FCM, most measures of milk composition, ruminal VFA, and plasma NEFA and BHBA concentrations were unaffected by niacin supplementation at this stage of lactation. Apparent digestibilities of most nutrients were greater when both sources of niacin were supplemented than when either source was supplemented separately. Duodenal nicotinic acid concentrations were higher for cows supplemented with nicotinamide than for cows receiving nicotinic acid, but the opposite was true for nicotinic acid concentrations in plasma. The results of both experiments indicated that nicotinamide was converted rapidly to nicotinic acid by microorganisms in the reticulorumen. Supplementation with either nicotinic acid or nicotinamide effectively can increase the amount of nicotinic acid available to the cow; however, some source effects remain to be explained.