Vitamin B12 requirements are exceedingly small; an adequate allowance is only a few µg per kg of feed, making it the most potent of vitamins. Poultry species requirements vary from 3 to 10 µg per kg (1.4 to 4.5 µg per lb) of feed (NRC, 1994).
Squires and Naber (1992) supplemented a corn-soybean diet for laying hens at control (no supplementation) or one, two or four times the NRC requirement for vitamin B12. Egg production was reduced after 12 weeks on the diets in the hens fed the two lowest vitamin B12 intakes. As vitamin B12 intake increased, shell thickness decreased and egg weight, hen weight, and hatchability increased. Maximum egg production, egg weight, hen weight, and hatchability were obtained when the diet contained 8 µg per kg (3.64 µg per lb) of vitamin B12.
The vitamin B12 requirements of various species depend on the levels of several other nutrients in the diet. Excess protein increases the need for B12 as does performance level. The B12 requirement seems to depend on the levels of choline, methionine and folic acid in the diet, and B12 is also interrelated with ascorbic acid metabolism (Scott et al., 1982). Sewell et al. (1952) showed that B12 has a sparing effect on the methionine needs of the pig. A reciprocal relationship occurs between B12 and pantothenic acid in chick nutrition, with pantothenic acid sparing the B12 requirement. Dietary ingredients may also affect the requirement, as wheat bran has been shown to reduce availability of vitamin B12 in humans (Lewis et al., 1986). Propionic acid is often used as a feed preservative, and propionic acid is known to increase the need for vitamin B12.
Dietary need depends on intestinal synthesis and tissue reserves at hatching. Intestinal synthesis probably explains frequent failures to produce a B12 deficiency in pigs and rats on diets designed to be B12-free. The deficiency can be readily produced in rats, however, when coprophagy is prevented completely (Barnes and Fiala, 1958). Coprophagous animals, including poultry, on deep litter receive excellent sources of B12 from microbial fermentation. Litter would be a less valuable source of vitamin B12 under cold conditions, where bacterial numbers are greatly reduced. Poultry obtain some vitamin B12 by direct absorption of the vitamin produced by bacterial synthesis in the intestine (NRC, 1994), but the amount derived from this source is small and not reliable.