An experiment was carried out to evaluate the selenium-vitamin E and night time or day time feeding on performance of Holstein dairy cows in heat stress. A total of 20 Holstein dairy cows (86±10 days postpartum and 600±20 kg BW) were randomly assigned to 4 treatments in a 2×2 factorial design and received a ration according to National Research Council (NRC) 2001 formulation. All dairy cows were divided into four groups, two groups were fed day time (5 cows for each group) and two groups were fed night time. One group from each day time and night time feeding groups was injected intramuscularly with 10 ml from a solution of selenium-vitamin E (each ml contains 0.5 mg selenium as sodium selenite and 50 mg vitamin E as dl-α-tocopheryl acetate) every 2 week. Night time feeding with selenium-vitamin E group was observed to have higher feed intake (kg DM/day), milk yield and milk persistency than other treatments (p>0.05). Night time feeding group had a higher dry matter intake (DMI) (kg/weight metabolic) than day time feeding group (p<0.05). The rectal temperature and respiration rate between treatments was not significant, but the selenium-vitamin E group was observed to be higher rectal temperature and less respiratory rate than non selenium-vitamin E group (p>0.05). The respiration rate for day time feeding group was observed to be higher than night time feeding group (p>0.05). Also blood plasma parameters, NH3-N and pH of rumen fluid were not affected by the treatments. Although night time feeding with selenium-vitamin E group in comparison of other treatments had no significant effect on milk production, DMI, blood plasma parameters, rumen fluid and physiologic condition, but it seems the performance of dairy cow for night time feeding with selenium-vitamin E can alleviate the effects of heat stress.

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