Wide variations in niacin requirements have been reported. Cunha (1982) lists a number of factors that influence niacin requirements: (1) genetic differences that can influence niacin needs; (2) selection for poultry with increased production levels; (3) the ability to synthesize niacin from tryptophan; (4) increased stress and subclinical disease level on the farm because of closer and more frequent contact between poultry in confinement; (5) trend toward more intensified operations, which may lessen opportunity for coprophagy; (6) newer methods of handling and processing feeds, which may affect niacin and tryptophan level and availability; (7) various nutrient interrelationships including amino acid imbalances; and (8) molds and antimetabolites in feeds that can increase certain niacin and tryptophan needs. Blair (1993) also suggests that increased stress associated with intensive production practices and disease influences niacin requirements. Rapidly growing broiler chicks require more niacin because of the faster gains of modern strains of birds as compared with more historical populations.
Animal species differ widely in ability to synthesize niacin from tryptophan. From a variety of experiments, approximately 60 mg of tryptophan is estimated to be equivalent to 1 mg of niacin in humans, while the rat is more efficient at a conversion rate of 35 to 50 mg of tryptophan required. Baker et al. (1973) suggested a conversion factor of 45 mg of tryptophan to 1 mg niacin in the young chick. Conversion efficiency is probably due to inherent differences in liver levels of picolinic acid carboxylase, the enzyme that diverts one of the intermediates (2-amino, 2-acroleylfumaric acid) toward the glutaryl-CoA pathway instead of allowing this compound to condense to quinolinic acid, the immediate precursor of nicotinic acid. Picolinic acid carboxylase in livers of various species has a very close inverse relationship to experimentally determined niacin requirements. Iron deficiency reduced tryptophan utilization for niacin synthesis (Oduho et al., 1994). For chickens, tryptophan conversion to niacin activity during an iron deficiency was a ratio of 56:1, but with an iron-adequate diet, 42:1.
Chicks have a conversion ratio of tryptophan:nicotinic ratio of 45 to 50:1 (Baker et al., 1973), while turkeys, with higher levels of picolinic acid, have conversion rates of 102:1 to 119:1 (Ruiz and Harms, 1988). The duck has a very high niacin requirement (approximately twice as high as chickens), with considerably higher levels of picolinic acid carboxylase activity (Scott et al., 1982). Chen et al. (1996) recently compared efficiency of conversion of tryptophan: nicotinic acid between ducklings and chicks. The results indicated tryptophan to nicotinic acid ratios of 181:1 for Pekin ducklings, 172:1 for Mule ducklings and 47:1 for broiler chicks. Liver picolinic acid carboxylase (PAC) activity was four to five times higher in ducklings than in chickens.
The NRC (1994) recommends from 10 to 65 mg of niacin per kg (4.5 to 29.5 mg per lb) of feed for various classes of poultry. For turkeys, the requirement is quite wide, varying from 40 to 60 mg per kg (18.2 to 27.3 mg per lb). Harms et al. (1988) reported that 23.6 mg per kg (10.7 mg per lb) of niacin from a corn-soybean diet was sufficient for maximum egg production and hatchability in turkeys. However, body and egg weights were significantly increased when 8.4 and 16.7 mg per kg of niacin were added (3.8 and 7.5 mg per lb) to the diet, respectively. Wen-Jie et al. (1995) suggested that 60 mg per kg (27.3 mg per lb) of niacin would be adequate for broiler chickens fed on a diet containing 0.25% tryptophan and 28.3 mg per kg niacin (12.9 mg per lb), respectively, from hatching to four weeks old, and 0.22% tryptophan and 25.8 mg per kg niacin (11.7 mg per lb) at five to seven weeks old. In another study with broilers, the niacin requirement was determined to be 80 mg per kg (36.4 mg per lb) feed (Whitehead, 2000).