Vitamin B6 is one of the B vitamins that receives the least amount of attention when poultry rations are formulated. Because of its wide distribution in feedstuffs, nutritionists generally expect adequate levels in typical poultry diets. Evidence to date indicates that corn, soybean meal and other ingredients used to supply energy and protein in practical poultry diets provide the minimum requirement of vitamin B6. However, the bioavailability in corn and soybean meal ranges from only 45% to 65% (Hoffmann-La Roche, 1979).
Under certain conditions, vitamin B6 supplementation is warranted for practical growing and breeding diets for poultry and other monogastrics. Fuller et al. (1961) believe that while corn-soybean meal practical poultry breeder diets probably provide the minimum requirement for vitamin B6 to support hatchability, there is little margin of safety. In turkey breeders there is decreasing deposition of vitamins B6 and B12 in the egg with aging (Robel, 1983). Hatchability of turkey eggs decreased with increasing age of the breeder hen; perhaps this is related to the decreasing nutrient deposition in the egg that occurs as the hen ages. The amount of supplemental vitamin B6 recommended for monogastric species varies from 1 to 10 mg per kg (0.45 to 4.5 mg per lb) of diet depending on species, age, activity, stress of performance and field conditions (Bauernfeind, 1974). Reasons for needed supplementation of vitamin B6 include the following (Perry, 1978): (1) great variations in amounts of B6 in individual ingredients, (2) variable bioavailability of this vitamin in feed, (3) losses reported during processing of feed ingredients, (4) discrepancies between activity for test organisms and those for animals, (5) a higher vitamin B6 requirement due to a marginal level of methionine in the diet and (6) high-protein diets.
Variability of vitamin B6 in feeds depends on the sample origin, conditions of growth, climate, weather and other local factors. Yen et al. (1976) determined available vitamin B6 in corn and soybean meal using a chick growth assay. B6 in corn was found to be 38% to 45% available, and B6 in soybean meal, 58% to 65% available. There was little difference in availability between corn samples not heated and those heated to 120°C (248°F). However, corn heated to 160°C (320°F) contained significantly less available B6. Level of vitamin B6 contained in feedstuffs is also affected by processing and subsequent storage. In one report, a loss of 30% of B6 was observed in alfalfa meal during the coarse milling and pelleting processes (Bräunlich, 1974). Bioavailability of feedstuffs can be as low as 40% to 50% after heating.
Predominant losses of vitamin B6 activity in feedstuffs occur in the pyridoxal and pyridoxamine forms, with pyridoxine the more stable form. Supplemental vitamin B6 is reported to have higher bioavailability and stability than the naturally occurring vitamin. Naturally occurring vitamin B6 in retorted milk products exhibited only 50% of the bioavailability of synthetic B6 or B6 in formulas fortified with the vitamin prior to thermal processing (Tomarelli et al., 1955).