Much of the niacin in common feeds (plant sources) is bound in a form that is not readily digestible by animals. Although the bioavailability of niacin is 100% in soybean meal, it is 0 in wheat and sorghum and varies from 0 to 30% in corn. In calculating the niacin content of formulated diets, probably all niacin from cereal grain sources should be ignored or at least given a value no greater than one-third of the total niacin content.
Tryptophan, the immediate precursor of niacin in animals, is particularly low in corn. Soybeans and soybean meal and cottonseed meal contain moderate levels (0.5%), while fish meal and blood meal are highest in tryptophan, although these products normally make up only a small portion of the ration.
Addition of niacin to a milk replacer is recommended based on lack of synthesis in the preruminant animal. The suggested level is 9 to 18 mg per calf per day (Roche, 1997).
As discussed in the previous section there have been positive responses of cattle and sheep to supplementary niacin. Supplementation appears to be of greatest benefit to stressed animals, such as beef cattle entering the feedlot, feedlot cattle and lambs and dairy cows immediately postpartum, particularly if symptoms of ketosis are present. Recommended supplemental niacin levels are 1 to 2 g per day for beef receiving cattle or feedlot cattle, 100 to 500 ppm in the diet of feedlot lambs and 3 to 12 g per day for dairy cattle.
Niacin may be beneficial in the acclimation to non-protein nitrogen (urea) in feedlot rations. However, the stimulation of microbial protein production by niacin was greatest when natural protein rather than non-protein nitrogen was fed (Brent and Bartley, 1984). Kung et al. (1980) reported no interaction between supplemental niacin and protein sources of either soybean meal or urea in lactating dairy cows.
Research has shown some beneficial effects of niacin in preventing or treating ketosis and possibly fat-cow syndrome. For individual cows with positive urinary ketone tests, a level of 12 g per day is recommended. When used as a general dietary supplementation, a level of 6 g of niacin per day is suggested, starting two weeks before calving.
Commercially produced niacin is quite stable compared to most other vitamins. Synthetic niacin and niacinamide were stable in premixes with or without minerals for three months (Verbeeck, 1975). Gadient (1986) reports that niacin is stable to heat, oxygen, moisture and light. The retention of niacin activity in pelleted feeds after three months at room temperature and dry storage conditions should be 95% to 100%. Niacin stability is reduced by the presence of trace minerals and choline.
