Dietary vitamin E requirements of ruminants have not been clearly defined. Minimum requirements for beef cattle (NRC, 1996), dairy cattle (NRC, 1989) and sheep (NRC, 1985) are estimated to range from 10 to 60 IU per kg (4.5 to 27 IU per lb) of diet dry matter. One international unit (IU) of vitamin E activity is defined as 1 mg of dl-alpha-tocopherol acetate, now called: all-rac-alpha-tocopherol acetate.
Absolute vitamin E requirements are difficult to determine due to interrelationships with other nutrients. Dietary levels of PUFA directly affect the vitamin E requirement. The PUFA found in unsaturated oils such as corn oil, cottonseed oil, soybean oil, sunflower seed oil and linseed oil all increase vitamin E requirements (McDowell, 2000). This is especially true if these oils undergo oxidative rancidification or are in the process of oxidizing when consumed by the animal. Rancidification of dietary fats prior to consumption destroys vitamins E, A and biotin. Fats ingested during the process of oxidation can cause damage to body tissues during absorption and metabolism and reduce vitamin E stores (Scott et al., 1982).
Ruminants are partly protected from the effects of dietary PUFA as a result of rumen biohydrogenation. However, investigations have shown that PUFA, when fed in large quantities, can escape rumen hydrogenation (McMurray et al., 1980). McMurray et al. (1980) and Rice et al. (1981) showed that nutritional muscular dystrophy (white muscle disease) could be induced by rapid introduction of calves to lush pasture, which is high in PUFA content. The pathogenesis was associated with and reproducible by elevations in plasma linolenic acid, which increased threefold within three days of turnout. In addition to these effects, dietary PUFA can cause destruction of vitamin E in feed prior to its contact with rumen microorganisms.
Various forms of stress, including weather extremes, social dominance interactions, feed or water deprivation, hauling, handling, disease exposure, trauma and toxins, all contribute to increased vitamin E requirements. Beef stocker cattle previously fed a low vitamin E ration or kept on low quality pasture exhibit a stress syndrome commonly referred to as "buckling." Affected calves come off the truck or out of the processing chute with weakness in the rear quarters, buckling of the hocks and fetlocks and generalized shaking and quivering of skeletal muscles (McDowell, 2000). Necropsy reveals pale, white streaks in striated muscles of the hamstring and lower back and sometimes in the heart, diaphragm and intercostal muscles of the ribcage.
Handling and bleeding heifers periodically over a 10-day period resulted in a large decrease in the vitamin E content of red blood cells and a 62% decrease in neutrophil vitamin E levels (Nockels, 1996). Vitamin E supplementation increased immune response in stressed calves (Golub and Gershwin, 1985). Cottonseed meal is commonly fed to cattle upon arrival at feedlots. The combination of shipping stress and gossypol consumption may reduce tissue vitamin E levels, further reducing disease resistance and increasing susceptibility to muscle damage (myopathy).
Vitamin E and selenium requirements are interdependent. The relationship has been quantified to a certain degree for poultry. Chicks consuming a diet containing 100 IU per kg (45.5 IU per lb) vitamin E required 0.01 ppm selenium, while those receiving no added vitamin E required 0.05 ppm selenium (Thompson and Scott, 1969). Hogan et al. (1990) reported that both vitamin E and selenium improved bactericidal ability of bovine neutrophils of cows previously fed vitamin E and selenium-deficient diets, but that their effects were not strictly additive. Therefore, these nutrients appear to exert a sparing effect on each other’s requirement.
Politis et al. (1995, 1996) found that high levels of vitamin E supplementation (3,000 IU per day) just prior to calving improved white blood cell function in cows fed diets that were adequate in selenium. Factors such as the level of disease challenge and the overall stress load on cattle are likely to influence this relationship. Tissue storage of vitamin E and selenium further complicate the determination of minimum requirements of these nutrients. Short-term studies may fail to account for the effects of tissue nutrient stores and thereby underestimate dietary requirements for both nutrients.
Dietary vitamin E requirements of young beef and dairy cattle range from 15 to 60 IU per kg (6.8 to 27.3 IU per lb) of diet dry matter (NRC, 1989, 1996). Requirements have not been clearly defined with adult beef cattle (NRC, 1996), although the requirement has been suggested as 15 IU per kg (6.8 IU per lb) of diet dry matter for adult dairy cattle (NRC, 1989). There is a strong basis for the latter requirement to be increased in the upcoming NRC dairy cattle publication (Weiss, 1998). The latest NRC beef cattle publication (NRC, 1996) recommends that 400 to 500 IU of vitamin E should be fed to receiving and starting feedlot cattle. There is a growing distinction between minimum requirements and optimal fortification with vitamins in the scientific literature.
Data summarized by Agricultural Research Council (1980) indicated that the minimum requirements for vitamin E in the diet of growing or pregnant sheep were between 10 and 15 IU per kg (4.5 to 6.8 IU per lb) of dry matter. However, if dietary selenium levels are below 0.05 mg per kg (0.023 mg per lb), even 15 to 30 IU per kg (6.8 to 13.6 IU per lb) may prove inadequate. The sheep NRC (1985) recommends that lambs under 20 kg (44 lbs) live weight should receive 20 IU per kg (9.1 IU per lb) of diet dry matter, while lambs over 20 kg (44 lbs) and pregnant ewes should receive 15 IU per kg (6.8 IU per lb) of diet dry matter.