This study was designed and executed to determine the effects of monensin in preparturient diet on postpartum milk production, energy metabolism, and reproduction performance of Holstein dairy cows. Forty Holstein dairy cows on close-up period were randomly divided into monensin treated (300 mg/day in close-up ration, top dress) and control groups. Body condition score (BCS) was estimated three weeks before and three weeks after calving. Milk production and milk fat percentage were recorded in both groups within 3 weeks postpartum. Blood samples were collected from five randomly selected cows of each group three weeks after calving. Serum concentrations of insulin like growth factor-I (IGF-I), insulin, glucose, and beta-hydroxybutyrate (BHBA) were measured. Calving to the first observed oestrus interval and calving to conception interval were compared between two groups. The results of the experiment showed that loss of BCS (P=0.3), increase of milk production (P=0.9), and milk fat percentage (P>0.05) were not significantly different between two groups during the period of study. In addition, mean serum glucose concentration (P=0.001) and serum insulin concentration (P=0.01) in monensin group were significantly higher than control cows in the first week postpartum. Moreover, serum BHBA concentration did not significantly change in monensin group. Serum IGF-I concentration in monensin group was significantly higher than control group in three weeks postpartum (P<0.01). The present study indicated that monensin treatment decreased calving to the first observed oestrus interval (P=0.05) and calving to conception interval (P=0.002). In conclusion, supplementing the close-up ration can increase postpartum serum IGF-I concentration and prevent the increase of serum BHBA concentration. These may result in enhancement reproductive performance of high-producing dairy cows.

Key words: Energy metabolism; High-producing dairy cow; Monensin; Reproductive performance