Requirement for vitamin B6 has been found generally to depend on species, age, physiologic function, dietary components, the intestinal flora and other factors that are not yet fully understood. Vitamin B6 is produced by microorganisms of the intestinal tract of swine, but whether significant quantities are absorbed and utilized is in doubt. Animals practicing coprophagy would obviously be receiving vitamin B6 from this source. In early studies, Hughes and Squibb (1942) estimated the pyridoxine requirement to be between 0 and 5 mg per 100 lbs of body weight daily for pigs with an average beginning body weight of 30 pounds. Vitamin B6 requirements for swine generally vary from 1 to 2 mg per kg (0.5 to 0.9 mg per lb) of diet (NRC, 1998). In early-weaned pigs, Kosters and Kirchgessner (1976a) indicated that lower levels of B6 are required for satisfactory feed efficiency than for optimum growth. Feed required per kg of weight gain in their experiment was increased only at the lowest vitamin B6 supplementation level, 0.5 ppm. For maximum weight gain, Kosters and Kirchgessner (1976a) suggested that 2 ppm was sufficient from 3.5 to 10 kg body weight, while only 1.2 ppm was required from 10 to 21 kg body weight. Using the same weight ranges, Kosters and Kirchgessner (1976b) reported similar requirements for optimum feed intake in early-weaned piglets. Furthermore, feed consumption was stated to be as sensitive a criterion of B6 supply as was growth rate. Miller et al. (1957) evaluated the pyridoxine requirements of baby pigs and determined that between 0.75 to 1 mg pyridoxine per kg dry matter of synthetic milk diet was needed for the pigs' total well-being under the conditions of their studies. Sewell et al. (1964) found a slightly higher vitamin B6 requirement for early-weaned pigs than the values reported by Miller et al. (1957). Under the conditions of their experiment, Sewell et al. (1964) recommended 0.4 to 1.6 mg vitamin B6 per lb of feed and estimated that 0.8 mg vitamin B6 per lb of feed would be required for pigs 3 to 8 weeks of age. Recently, data by Woodworth et al. (1997) supported the addition of 2 to 3 g pyridoxine per ton of diet for maximum average daily gain and average daily feed intake on days 0 through 14 after weaning. They also recommended that SEW and transition diets contain 3 g of added pyridoxine per ton for practical applications. Matte (1995) suggested that the recommendations for water-soluble vitamins in swine are in many cases based on research that was reported decades ago and on deficiency syndromes. Furthermore, he emphasized that today's pigs are raised in different housing and under different feeding conditions and vary in genetics greatly from the pigs used in many of the early research studies. Matte (1995) indicated that the optimum pyridoxine level is probably twice the Agricultural Research Council (1981) level.
Pigs weighing 1 to 5 kg (2.2 to 11 lbs) require 2 mg vitamin B6 per kg (0.9 mg per lb) of feed, while pigs greater than 20 kg (44 lbs) and breeding animals require 1 mg per kg (0.5 mg per lb) of vitamin B6 in feeds (NRC, 1998). Breed of animal and environmental temperature have been shown to influence vitamin B6 requirements for some species (i.e., poultry and rats). Regarding ambient temperature, when rats were housed at 33°C, they needed twice as much vitamin B6 as when they were housed at 19ƒC (Braunlich, 1974)
Quantity of dietary protein affects requirement for vitamin B6 in both animals and humans. Vitamin B6 requirement is increased when high-protein diets are fed. For example, when feed contained 60% casein instead of 20%, the level of pyridoxine required by mice was three times as high (Miller and Baumann, 1945). A number of studies have suggested that amino acid imbalance has an adverse effect on vitamin B6 status, in that weight gain was depressed and survival was decreased when large amounts of a single amino acid were added to rat diets limited in the vitamin. High tryptophan, methionine and other amino acids increase the need for vitamin B6 (Scott et al., 1982). Matte et al. (1998) indicated that based on pyridoxine status and data concerning glycemia and insulinemia stimulated by enteric glucose in pyridoxine- supplemented early-weaned pigs, the optimal daily parenteral pyridoxine was 15 mg. However, in their study, a decrease in riboflavin status was pronounced in the piglets with the highest gastric tube feeding amount and greatest pyridoxine supply. Matte et al. (1998) therefore concluded that the optimal dietary level of pyridoxine that reproduces the effects of the parenteral pyridoxine should be determined, and in addition, effects on riboflavin status should be taken into consideration.
Certain feed antagonists, bioavailability of B6 in feeds and nutrients other than protein influence the B6 requirement. Niacin and riboflavin are needed for interconversions of different forms of vitamin B6, with an overdose of thiamin reported to produce vitamin B6 deficiency in rats (Driskell, 1984).
