There are severe limitations in relying on average tabular values of vitamins in feedstuffs. As an example vitamin E content of 42 varieties of corn varied from 11.1 to 36.4 IU per kg (5.0 to 16.5 IU per lb), a 3.3 fold difference (McDowell and Ward, 2008). The average vitamin levels in some common feed ingredients are presented in Table 4 for use as a reference in adjusting vitamin allowances for animal diets. These levels were derived mostly from published NRC data (NRC, 1982). However, values for vitamin E, vitamin B6 and biotin, determined by assay at Roche, are also included. Generally, these average values are based on only a limited number of assays and were not adjusted for bioavailability and variations of vitamin levels within ingredients.
Much of the data on which these values are based were published more than 30 to 40 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 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). Likewise, in more recent years pelleting temperatures have increased in order to better control Salmonella; one result is greater vitamin destruction (Gadient, 1986). 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 when formulating animal rations have been reported (Kurnick et al., 1972).
Vitamin levels from feedstuffs in simple rations are generally lower than levels 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 complex diets compared to simple diets, the “allowance-to-NRC value” in the complex ration is a higher ratio 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 best-cost diet formulation, the low levels of vitamins likely to be supplied by feedstuffs should be disregarded and adequate dietary vitamin fortification provided.
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