The following answer is excerpted from a presentation by Barry Bradford, dairy scientist at Kansas State University, at the Tri-State Dairy Nutrition Conference in April.
Q. Should I use caution when feeding dietary antioxidants to promote transition health?
A: Dietary antioxidants, notably vitamin E and selenium, are important for their ability to
contribute to reactive oxygen species neutralization, thereby impeding the progression toward inflammation...
Given the importance of antioxidants in modulating inflammation, it is not surprising that multiple studies have shown that supplementing vitamin E in excess of traditional recommendations decreases the incidence and severity of clinical mastitis (Smith et al., 1984; Weiss et al., 1990a). More recently, a meta-analysis showed that supplemental vitamin E is also effective at preventing retained placenta (Bourne et al., 2007)...
Although much of the literature on antioxidants in transition cows demonstrates positive effects, these nutrients must be used with caution. In an effort to maximize the odds of observing a response, most studies are designed with rather dramatic treatments; for example, one classic study (Weiss et al., 1990b) compared vitamin E intakes of 574 IU/day (no supplemental vitamin E) to 1474 IU/day (supplementing 88 IU/lb dry matter). In many such scenarios, the control group is fed a diet that is marginally deficient in the nutrient of interest. On most dairies, this is not the case. As a result, adding large amounts vitamin E, for example, can sometimes push the supply of the nutrient high enough to cause mild toxicity. Supplementing 3000 IU/day vitamin E to transition cows with adequate vitamin E status resulted in pro-oxidant responses, increasing markers of lipid peroxidation and the incidence of mastitis (Bouwstra et al., 2010).
Any treatment that alters oxidative balance should be evaluated carefully.