Wide variations in niacin requirements of animals have been reported. Cunha (1982a) identified a number of factors that influence niacin requirements in swine, including the following:
- Genetic differences that can influence niacin needs and selection for swine with increased production levels.
- The ability to synthesize niacin from tryptophan.
- Limited bioavailability of niacin in feeds.
- Increased stress and subclinical disease level on the farm because of closer and more frequent contact among pigs in confinement.
- Trend toward more intensified operations, which may lessen opportunity for coprophagy.
- Trend toward earlier weaning, which increases the need for higher niacin levels in milk-substitute diet (prestarter and starter feeds).
- Newer methods of handling and processing feeds, which may affect niacin and tryptophan level and availability.
- Various nutrient interrelationships, including amino acid imbalances.
- Molds and antimetabolites in feeds that can increase niacin and tryptophan needs.
Metabolic conversion of excess dietary tryptophan to niacin by all classes of swine except the newborn pig has complicated the determination of the niacin requirement (Luecke et al., 1948). Firth and Johnson (1956) estimate that each 50 mg of tryptophan in excess of the tryptophan requirement yields 1 mg of niacin; therefore, tryptophan can be a source of niacin in niacin-deficient diets. Swine niacin requirements generally range between 7 and 20 mg per kg (3.2 and 9.1 mg per lb) of diet (NRC, 1998). Firth and Johnson (1956) estimated the available niacin requirement for 1 to 8 kg (2.2 to 17.6 lbs) body weight pigs to be about 20 mg per kg (9.1 mg per lb) for a diet with no excess tryptophan. When tryptophan is fed to meet the requirement, Harmon et al. (1969) concluded, the weanling pig needs approximately 13.2 mg of available nicotinic acid per kg of diet. Harmon et al. (1970) reported that in comparison with unsupplemented pigs, young swine that received corn-based diets formulated to contain the marginal requirement for tryptophan had improved rate of gain and gain:feed ratio when either 0.01% l- or dl-tryptophan or 13.2 ppm nicotinic acid was added. Powick et al. (1947b) found that the level of nicotinic acid required for optimal growth of pigs between the ages of 3 and 9 weeks was 0.6 to 1.0 mg per kg live weight per day. However, Copelin et al. (1980) found no benefit of 5, 10 or 22 mg of niacin per kg diet versus the control with no added niacin when growing-finishing swine were fed corn-soybean meal diets formulated to provide 17.46% and 14.66% crude protein in the grower (up to 60 kg) and finisher (to 100 kg) diets, respectively. Findings of Yen et al. (1978) support the results of Copelin et al. (1980). Yen et al. (1978) fed 0, 5, 10 or 15 ppm of niacin in a typical fortified corn-soybean meal diet that contained 4.45% or more soybean meal (44% protein) and found no benefit of niacin on finishing-pig performance. There is limited information available for establishing the niacin requirement of pregnant and lactating sows; an estimated requirement of 10 mg per kg (4.5 mg per lb) has been extrapolated from growing-pig data (NRC, 1998). Goodband et al. (1987) evaluated the effects of supplemental niacin on sow and litter performance through two parities. Sows received a corn-soybean meal diet that provided the following niacin supplementation during gestation and lactation, respectively: 50 and 100, 250 and 500 or 500 and 1000 mg per day. Results of this study suggested a tendency for higher pig survivability for primiparous sows fed additional niacin. Sows fed additional niacin also weaned heavier pigs. Goodband et al. (1987) suggested that these results may have been due to improved milk production of sows receiving additional niacin. Similar results have been reported (Hutjens, 1990) in dairy cows supplemented with 6 to 10 g of niacin daily with respect to increases in total milk yield and fat content of milk. Goodband et al. (1987) suggested that optimum daily niacin requirements for sows based on sow and litter performance appear to be 250 to 500 and 500 to 1000 mg per day for gestation and lactation diets, respectively. Not all research findings have supported the addition of niacin beyond that available in typical sow rations. Ivers et al. (1993) concluded that a corn-soybean meal-oat diet with 12.8% crude protein provides adequate niacin without further supplementation with crystalline niacin for gestating and lactating sows. The basal diet contained 23 mg of niacin per kg. Sows assigned to the treatment group received 33 mg of supplemental niacin per kg added to the basal diet, and their performance did not differ from those provided just the basal diet. Ivers et al. (1993) concluded that adequate niacin was available from the feed ingredients in addition to niacin from excess tryptophan in the diet or niacin synthesis by intestinal tract bacteria. The results of investigations by Goodband et al. (1994) suggest that sow productivity may influence the niacin requirement. In their recent report, primiparous sows fed 250 or 500 mg of niacin per day versus 50 or 100 mg of supplemental niacin per day during gestation and lactation, respectively, had increased pig survival from birth to weaning and increased pig and litter weaning weights. However, Goodband et al. (1994) reported that parity II sows receiving niacin supplements showed no improvements in sow or litter performance.