THE EFFECT OF ZERANOL IMPLANTS ON GROWTH AND PREGNANCY LOSS IN BEEF HEIFERS

Live weight, average daily gain (ADG) and reproductive performance wer e compared between heifers implanted with zeranol at birth, 100 and 20 0 d of age (n = 54) and those not implanted (n = 50). Zeranol implanta tion resulted in increased (P < 0.05) ADG from birth to weaning, weani ng weight, ADG from weaning to breeding, breeding weight and calving w eight but did not affect (P > 0.05) calf birth weights, incidence of d ystocia, calf survival or gestation lengths. Zeranol implants did not affect (P > 0.05) the incidence or magnitude of the pre-ovulatory LH s urge 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 a t 72 d (P < 0.10), and was different at 133 d (P < 0.05), with implant ed heifers having a lower pregnancy rate than non-implanted heifers. B etween day 72 and day 133 after bull entry there was a 10 % embryonic or fetal mortality rate in zeranol-implanted heifers, which differed f rom the 0% loss in non-implanted heifers (P < 0.05). We conclude that serial implants of zeranol from birth reduced the pregnancy rate in he ifers and that this effect was due to early fetal loss.