To compare two oestrus synchronisation protocols in beef heifers presynchronised with two prostaglandin F2α (PG) treatments, 36 h after the second PG treatment one group (hCG-OB group; n = 112) received 250 IU of hCG and 0.5 mg of oestradiol benzoate (OB) followed by fixed time artificial insemination (FTAI) 30 h later. Additional oestrus detection (OD) and artificial insemination (AI) was done in this group for 3 days after FTAI; heifers rebred during this period were considered open to the FTAI. The Control group (n = 112) received only OD and AI for 6 days after the second PG treatment. Follicular size was measured at the time of AI in a subsample of heifers (59 in the hCG-OB group and 34 in the Control group). Results were evaluated separately either considering heifers pregnant at FTAI or those pregnant at FTAI + 3-day OD + AI. The FTAI conception rate was 45.5 vs 76.5% for the Control group (P < 0.0001) and pregnancy rates were 45.5 and 67.0% (P = 0.0014); FTAI + OD + AI conception rates were 63.4 and 76.2% (P = 0.0404) and pregnancy rates 63.4 and 62.8% (P = 0.5719) (hCG-OB and Control groups, respectively). There was a significant correlation (r = 0.713, P < 0.01) between follicle size at FTAI and pregnancy in the hCG-OB group, but not in the Control group (r = 0.3465, P > 0.1). In conclusion, acceptable synchronised conception rate and pregnancy rate were obtained by using this protocol, and the results were improved by complementing it with OD + AI for 3 days after FTAI.

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