Changes in life events and environment, such as parturition, milk production, and heat stress, substantially increase demands on the cow including her oxygen requirements. These increased requirements for oxygen usually result in increased production of troublesome reactive oxygen compounds. These must be neutralized with an anti-oxidant such as vitamin E. 

At the University of Florida, we increased the daily intake of supplemental vitamin E from 1000 to 3000 international units (IU) during the close-up nonlactating period and from 500 to 2000 IU after calving. The 1000 and 500 IU amounts are what is recommended by the National Research Council. In addition, the cows were kept in shade only or with shade, fans, and sprinklers during the last 4 weeks of pregnancy. After calving, all cows were provided with shade, fans, and sprinklers. Milk yield and feed intake were measured for the first 15 weeks.

The older cows responded differently than the firstcalf heifers. If older cows were offered shade before calving, 3.5% fat-corrected milk production increased from 79.4 lb per day to 87.9 lb per day. Yet feeding additional vitamin E to the older cows without fans and sprinklers had the same effect as cooling the cows before calving; that is, 3.5% fat-corrected milk increased from 79.4 lb per day up to 87.0 lb per day. No benefit of feeding extra vitamin E was detected if older cows were evaporatively cooled before calving. Therefore milk yield by older cows was the same if 1) they were evaporatively cooled without increased supplementation of vitamin E or 2) they only had shade before calving but were fed extra vitamin E. These increased amounts of milk yield were supported by increased amounts of feed intake. 

The story was much different for first-calf heifers. Production of 3.5% fat-corrected milk was reduced if they were fed vitamin E above NRC recommendations regardless of prepartum cooling method. Milk yield dropped from 61.1 to 49.7 lb per day by feeding extra vitamin E to heifers only given shade. But milk also dropped from 59.5 to 54.4 pounds per day if extra vitamin E was fed to heifers cooled with fans and sprinklers before calving.

Why such a difference in response between heifers and cows? Based upon lower plasma concentrations of nonesterifed fatty acids (NEFA), less loss of body weight, and less negative energy balance, first calf heifers were under less stress postpartum than were older cows. Feeding 3 to 4 times the recommended amount of the antioxidant vitamin E to these lowerstressed heifers may have caused vitamin E to form many tocopherol radicals that damaged cell membranes and hurt performance rather than act as an antioxidant and help performance as it did with the older cows. The combination of increased heat stress before calving and greater metabolic stress due to greater milk production postpartum may have created a situation in which the requirement for an antioxid.