Reduced response to an estrous induction program in postpartum beef cows treated with zilpaterol and gaining body weight.

Abstract In beef cows, reduced energy intake delays first ovulation postpartum and is associated with lesser insulin, IGF-I and leptin concentrations. However, the close relationship among these hormones mask their individual roles in the reinitiation of ovarian activity. A β-adrenergic receptor agonist (βAR) was used to increase body condition score (BCS) and yet reduce body fat and leptin serum concentration to determine the specific role of leptin in the postpartum ovarian activity. Beef cows ( n =77) with BCS 3.1±1.4 received 2kg/day of feed containing 0 or 0.15mg/kg of zilpaterol (a synthethic βAR), for 33 days. Estrus was induced with a progestin implant applied for 9 d and cows in estrus were bred by artificial insemination (AI). Zilpaterol administration increased ( P P P P =0.05) and 56.5% ( P =0.007), respectively, in zilpaterol-treated cows. Logistic regression estimates for BCS ( P =0.016) and IGF-I concentration ( P =0.03) were positively related with the occurrence of luteal activity. In addition, whilst back fat ( P =0.009) had a positive effect on luteal activity, leptin concentration did not show a significant relationship. In conclusion, despite an increase in body weight and a positive change in BCS, the reduction in insulin and IGF-I concentrations, associated with βAR treatment, reduced the response to induction of estrus. However only IGF-I, but not leptin or insulin, significantly influenced the odds for the occurrence of luteal activity after estrous induction in cattle with poor BCS.