The vitamin E levels of corn can vary by more than 100 percent (Table 1). Factors affecting the vitamin content of any feedstuff can include variety, geography, cropping practices and quality of finished crop.
Furthermore, harvesting and processing conditions can reduce vitamin levels significantly. The vitamin content of corn is drastically reduced when harvest months are not conducive to full ripening. If corn has been subjected to alternate periods of freezing and thawing while it contains a high amount of moisture, fermentation in the corn kernels will result in vitamin loss, especially of vitamin E.
Similarly, good green forage as found in pastures is an excellent source of vitamin E for ruminants and horses, but the vitamin E content decreases with forage maturation. All grasses also lose about 30 to 60 percent of their vitamin E content during drying. When subjected to rain, excessive sunlight or barn storage at high temperature, they can lose even more. In general, moisture, heat, light, oxidation-reduction and rancidity will affect any feed ingredient's vitamin content significantly.
When calculating the vitamin levels in feedstuffs, another issue that arises is bioavailability, the amount that is available for use by the animal in its biological function. For example, the bioavailability of choline is 100 percent in corn but 60 to 75 percent in soybeans, and the bioavailability of niacin is 100 percent in soybeans but 0 to 30 percent in corn. Thus in adjusting fortification levels in diets, it is important to know the chemical form of the vitamins as well as total content.