Effects of repetitive norgestomet treatments on pregnancy rates in cyclic and anestrous beef heifers

Sixty-eight 12- to 14-month-old crossbred beef heifers averaging 285 kg were assigned at random to treated (n = 35) and control (n = 33) groups to evaluate the use of repetitive norgestomet treatments. Treated heifers received an ear implant containing 6 mg norgestomet on two occasions 16 days apart. Injections of 3 mg norgestomet and 5 mg estradiol valerate (EV) were given intramuscularly the same day as first implantation (Syncro-Mate-B). Implants were removed after eight days. Four bulls were then placed in each of two pastures containing half of the treated and half of the control heifers for 24 days after the time of the first implant removal. Progesterone concentrations from blood samples collected prior to the first treatment were used to determine reproductive status. The overall pregnancy rate 64 days after first implant removal for treated anestrous heifers (61%; 14 23) was similar (p > 0.25) to untreated (73%; 11 15) and treated (75%; 9 12) cyclic heifers, but higher (p < 0.1) than for untreated anestrous heifers (33%; 6 18). This treatment advantage resulted from an increased (p < 0.01) pregnancy rate after the second implant removal. In summary, repetitive norgestomet treatments enhanced pregnancy rate in anestrous heifers within a 24-day breeding season.