Much of the data on which these values are based were published more than 20 to 30 years ago. They may not reflect the changes in genetic characteristics, handling and storage of crops, cropping practices and processing of meat and grain by-products that have occurred over the years. Changing processing methods can greatly alter vitamin feed levels. As an example, with changes in sugar technology, literature values for pantothenic acid content of beet molasses have decreased from 50 to 110 mg per kg (22.7 to 50 mg per lb) in the 1950s to about 1 to 4 mg per kg (0.45 to 1.82 mg per lb) more recently (Palagina et al., 1990). In addition, values for some vitamins were not determined by current, more precise assay procedures. Additional information on the limitations of using average values of vitamins in feedstuffs in formulating animal rations has been reported (Kurnick et al., 1972).
Vitamin levels from feedstuffs in simple rations are generally lower than in complex rations. The currently used simpler rations (e.g., corn-soybean meal diets) for animals exclude or contain lower amounts of the more costly vitamin-rich ingredients. To "fill in the gaps" resulting from the reduced amounts of vitamins supplied by feedstuffs, the vitamin fortification levels in these simpler diets should be increased.
Because higher levels of many vitamins are supplied by the feedstuffs in the complex diets compared to simple diets, the "allowance-to-NRC value" in the complex ration is higher ratios for these vitamins. Vitamin fortification of the simple diet should be increased to provide allowances necessary to achieve optimum performance.
The vitamin levels for feedstuffs in these diets were not adjusted for bioavailability and variations of vitamin levels among feedstuffs or within a given feedstuff class. Additionally, vitamin losses can occur during processing and storage of feedstuffs and finished feeds. Therefore, levels of various vitamins from feedstuffs may not adequately meet the vitamin needs of animals when the complete diet is fed.
Since ingredient changes are frequent and unpredictable in computerized least-cost diet formulation, the low levels of vitamins likely to be supplied by feedstuffs should be disregarded and adequate dietary vitamin fortification provided.
