Manipulation of antioxidant status fails to improve fertility of lactating cows or survival of heat-shocked embryos.

Experiments were conducted to test whether enhancement of antioxidant status could improve fertility and milk yield in dairy cows and resistance of cultured embryos to heat shock. Three experiments in three herds were performed to evaluate the effect of multiple intramuscular injections of 500 mg of vitamin E and 50 mg of selenium at 8 to 21 d before expected calving and at 30 and 80 d postpartum on reproduction of lactating Holstein cows. Vitamin E and selenium injections did not improve reproductive function or milk yield in any of the studies. The predicted 305-d milk yield (averages of least-squares means across treatments) were: 9478, 7073, and 10,204 kg projected 305-d milk for experiments 1, 2, and 3, respectively. Percentages of cows pregnant at first service were 30, 16, and 24% in experiments 1, 2, and 3, respectively. Three studies were performed to test whether vitamin E improved development of cultured bovine embryos exposed to heat shock. Heat shock of 41 degrees C at the two-cell stage reduced development to the blastocyst stage, but culture with 100 microM vitamin E did not reduce effects of heat shock on impaired development. For example, 9 h at 41 degrees C reduced blastocyst development from 51.2 +/- 3.3% to 3.4 +/- 3.3% in the absence of vitamin E and from 54.0 +/- 3.3% to 5.2 +/- 3.3% in the presence of vitamin E. Development of morulae to the blastocyst stage was not compromised by culture at 41 degrees C for 9 h. Additionally, there was no overall effect of vitamin E on morula development. In conclusion, multiple injections of vitamin E and selenium at the administered levels did not improve postpartum fertility nor milk yield of lactating Holstein cows in three different herds, and there was no direct thermoprotective effect of vitamin E for cultured, heat-shocked embryos.