In view of the nutritional importance of vitamins A, D and E, many commercial vitamin producers have succeeded in enhancing stability of these vitamins in two ways: (a) by mechanical means of enveloping minute droplets of the vitamin or vitamins in a stable fat or gelatin, forming small beads and thus preventing most of the vitamin from coming into contact with oxygen until it is digested in the animal intestinal tract, and (b) through use of effective antioxidants that markedly prolong the induction period that precedes active vitamin oxidation. The "stabilized" beadlet containing an effective antioxidant will protect these vitamins for storage periods up to six months without much loss of vitamin potency (Hoffmann-La Roche, 1988). Instability of vitamin D in peroxidizing diets is often overlooked. Studies have shown that very high dietary levels of vitamin D are completely destroyed in diets containing high levels of peroxidizing PUFAs, and chicks fed such diets suffer severe rickets by three weeks of age (Scott, 1973). Rickets was prevented by a normal level of vitamin D when the diet was supplemented with the antioxidant ethoxyquin at a level of 57 mg per kg of diet (26 mg per lb).
Stress factors affecting vitamin stability during manufacture and storage of a custom premix may include heat, oxygen, moisture, oxidation, reduction, trace minerals and pH. Research tends to indicate that moisture is probably the most important factor in causing vitamin stability problems in the feed industry (McGinnis, 1988). Some vitamins, such as riboflavin, niacin, biotin, pantothenic acid and choline, generally have excellent stability in custom premixes. Other vitamins, such as vitamin A acetate, vitamin D and vitamin E acetate, are available and added as stabilized forms in custom premixes. Vitamin K, unstabilized vitamin A, unstabilized vitamin D, thiamin, folic acid, vitamin C, vitamin B6 and vitamin B12 have poor stability in custom premixes under various stress conditions (Frye, 1978).
Most vitamins are supplemented in dry forms for feed supplementation. In addition, liquid forms of some vitamins have been developed for certain applications. When an animal¹s feed intake is insufficient to provide adequate vitamin intake, but where there is good control over water consumption, special dry or liquid water-dispersible vitamins A, D3 and E products and multivitamin stress packs are available to add to drinking water. These will disperse in cold water into a fine emulsion with desirable physical, taste and chemical stability characteristics for short time use. These vitamin forms are useful in drinking water for pets and other animals.
Because of the high variability and unknown bioavailability of vitamins in feeds, feed manufacturers have come to rely to a large extent on commercially synthesized vitamins. In practice, feed manufacturers usually ignore to a certain degree the contribution of many vitamins in feedstuffs and provide complete vitamin supplementation. Bauernfeind (1969) summarized the advantages of synthetic vitamins in animal feeds as follows:
- Biological and physical characteristics are known; potency is uniform; stability is adequate; supply is usually unlimited.
- Weight added per ton to the animal diet is small, hence, adjustments upward or downward or in different ratios can be made without upsetting the remainder of the diet.
- Standard and custom multivitamin premixes are formulated for quick addition to specific diets for a given species of animal for a defined production objective with assurance of known diet values.
- Cost is economical; hence, vitamin restrictions of individual natural feed ingredients can be removed from computer programs, thus increasing the flexibility of programming for least-cost energy and protein needs and shortening computer operation time.
- Assay costs of determining variability of natural vitamin content of feedstuffs can be decreased by supplementing with the chemically produced nutrients.
- Assay costs of determining the vitamin content of the final mixed diet can be decreased since assay of one or two components of the premix will give confidence of the mixing adequacy of all of the premix vitamins in the final diet.
- Use of chemically prepared vitamins eliminates unknowns of, for example, naturally occurring ingredients, seasonal nutrient variation and physiological nutrient availability.
- Synthesized vitamins have versatility in form and applications adaptability to dry feeds, liquid supplements, drinking water, drenches, parenterals, boluses, tablets and capsules.