Mastitis and oxidative stress in vitamin E supplemented dairy cows

The research described in this thesis evaluated the effect of vitamin E supplementation under field conditions on the udder health of Dutch dairy cows. Additionally, it investigated the mechanism by which vitamin E influenced oxidative stress, especially during the dry period. Moreover, it investigated whether fat metabolism during the dry period influenced oxidative stress. The main finding presented in this thesis is the adverse effect of vitamin E supplementation on udder health in early lactation; both subclinical and clinical mastitis incidences were significantly higher in dairy cows that received a daily supplement of 3,000 IU/cow/day during the dry period compared to cows that received 135 IU. This appears to be the first time that an adverse effect of vitamin E was found in dairy cows. It was both a completely unexpected outcome, and one that highlights the importance of awareness of possible adverse effects of vitamin E. The adverse effect of vitamin E might be explained by the fact that high levels of vitamin E can, under some conditions, increase oxidative stress. High levels of oxidative stress are, in turn related to a higher risk of clinical mastitis. Thus, for some of the cows in this study, oxidative stress from high amounts of vitamin E during the dry period resulted in more mastitis. Therefore, vitamin E supplements are, for some dry dairy cows, not needed. Moreover based on all the results gathered on oxidative stress, it can be concluded that gaining insight into the mechanism by which vitamin E influences oxidative stress is quite difficult. Even in well-controlled experiments, no general effect of vitamin E supplementation was observed. In addition, while it seems that blood levels of vitamin E and the ratio of vitamin E to cholesterol are very reliable, it still remains difficult to define an oxidative status of the cow only based on blood levels of different oxidative stress related parameters. Because the clinical trial showed an adverse effect of vitamin E, it remains complicated to evaluate whether the assumed link between vitamin E supplementation and (udder) health improvement is through reduced oxidative stress is correc