Vitamin B6 is one of the B vitamins that is least likely to be deficient in swine. Because of its wide distribution in feedstuffs, nutritionists generally expect adequate levels in typical swine diets. Evidence to date indicates that corn, soybean meal and other ingredients used to supply energy and protein in practical swine diets provide the minimum requirement of vitamin B6. However, the bioavailability of vitamin B6 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 swine. 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 age, activity, stress of performance and field use experience (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 ingredients, (3) losses reported during processing of 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. Matte (1997) indicated that pyridoxine requirements increase with more rapid growth rates. He reported that for weaned piglets, the dietary levels of B6 optimal for growth performance are two to five times greater than the requirements for growing-finishing pigs based on a variety of data (Kosters and Kirchgessner, 1976a; NRC, 1998; Bretzinger, 1991). Matte (1997) also stated that as early-weaning practices increase, optimum B6 levels may be even higher. Based on pyridoxal-5-phosphate measurements in plasma and red blood cells, Matte (1997) indicated that a high post-weaning requirement for B6 in piglets may be related to the reduced quality and increased quantity of protein in feed versus sows' milk. This in turn would cause more interconversion and oxidation of amino acid reactions, which depend on pyridoxal phosphate.
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 grow assay. B6 in corn was found to be 38% to 45% available, and B6 in soybean meal, 58% to 62% available. It is probably equally available for the pig. There was little difference in availability between corn samples not heated and those heated to 120ƒC. However, corn heated to 160ƒC contained significantly less available B6. The level of vitamin B6 in feedstuffs is also affected by processing and subsequent storage. In one report a loss of 30% of B6 in alfalfa meal during the coarse-milling and pelleting processes was observed (Braunlich, 1974). Bioavailability can be as low as 40% to 50% after heat processing of feedstuffs. Sewell et al. (1964) investigated the effect of corn oil on vitamin B6 requirements and found no measurable influence of corn oil on the requirement. Roth-Maier and Kirchgessner (1997) examined the effects of wheat bran and alfalfa meal supplements on vitamin B6 metabolism in sows provided with a suboptimal B6 supply. In comparison with cellulose, bacterially fermentable substrates from wheat bran induced higher bacterial vitamin B6 synthesis. Fecal vitamin B6 excretion was linearly increased with increasing fibrous supplementation.
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 a higher bioavailability and stability than the naturally occurring vitamin. Naturally occurring vitamin B6 in retorted milk products exhibited only 50% of the bioavailability of synthesized B6 or B6 in formulas that were fortified with the vitamin prior to thermal processing (Tomarelli et al., 1955).
The recovery of vitamin B6, as pyridoxine hydrochloride, in a multivitamin premix not containing trace minerals was 100% even after three months of storage at 37ƒC. However, stability in a premix containing trace minerals was poor, with only 45% recovery after three months at 37ƒC (Adams, 1982). Verbeeck (1975) found vitamin B6 to be stable in premixes with minerals as sulfates. However, if minerals in the form of carbonates and oxides are used, 25% of the vitamin can be lost over a three-month period. Stress agents such as choline chloride help catalyze this destruction. Gadient (1986) considers pyridoxine to be very sensitive to heat, slightly sensitive to moisture and light, and insensitive to oxygen. Retention of B6 activity in pelleted feeds after three months at room temperature should be 80% to 100% as a general rule.