The effect of zeranol implants on growth and pregnancy loss in beef heifers

Abstract

Live weight, average daily gain (ADG) and reproductive performance were compared between heifers implanted with zeranol at birth, 100 and 200 d of age (n = 54) and those not implanted (n = 50). Zeranol implantation resulted in increased (P < 0.05) ADG from birth to weaning, weaning weight, ADG from weaning to breeding, breeding weight and calving weight but did not affect (P > 0.05) calf birth weights, incidence of dystocia, calf survival or gestation lengths. Zeranol implants did not affect (P > 0.05) the incidence or magnitude of the pre-ovulatory LH surge or the number of heifers that ovulated between 48 and 168 h after cloprostenol injection. Pregnancy rate 42 d after bull entry did not differ between the two groups (P > 0.10), tended toward a difference at 72 d (P < 0.10), and was different at 133 d (P < 0.05), with implanted heifers having a lower pregnancy rate than non-implanted heifers. Between day 72 and day 133 after bull entry there was a 10% embryonic or fetal mortality rate in zeranol-implanted heifers, which differed from the 0% loss in non-implanted heifers (P < 0.05). We conclude that serial implants of zeranol from birth reduced the pregnancy rate in heifers and that this effect was due to early fetal loss. Key words: Zeranol, embryonic/fetal mortality, bovine, pregnancy rate, ultrasonography