This study compared pregnancy per TAI (P/TAI) between sex-selected and conventional semen in cyclic beef heifers subjected to the modified (without initial GnRH) 5-day Co-synch protocol and evaluated the usefulness of an oestrus detection (ED) to identify heifers that were most likely to conceive. Heifers (n = 1034; 13 to 15 months of age) at three locations were scanned by ultrasonography to determine cyclicity (presence of corpus luteum) and reproductive tract normalcy. Cyclic heifers (n = 940) received a 1.38-g progesterone-releasing device (CIDR; Zoetis Animal Health, Florham Park, NJ, USA) on Day 0, CIDR removal and 500 μg of cloprostenol (Estroplan; Vetoquinol NA Inc., Edmonton, AB, Canada) on Day 5, and 100 μg of GnRH (Fertiline; Vetoquinol NA Inc.) along with TAI on Day 8 (72 ± 1 h after CIDR removal). On Day 5, heifers received ED patches (EstrotectTM; Estrotect Inc.) that were scored from 0 to 3, based on colour change between initial application and Day 8; 0 = unchanged, 1 = ≤50% colour change, 2 = >50% colour change, 3 = missing. Heifers were assigned by ear-tag number to be inseminated with frozen–thawed sex-selected or conventional semen from either of two sires available commercially. Pregnancy diagnosis was performed by transrectal ultrasonography 28 (herd A) or 48 (herds B and C) days after TAI. Data were analysed using the PROC GLIMMIX in SAS (SAS Institute Inc., Cary, NC, USA), with herd included as a random effect. The CIDR retention rate was 98%; P/TAI in heifers that lost the CIDR was 21%. Pregnancy per TAI [% (no.)] did not differ (P > 0.05) between sires [58.4 (426) v. 58.0 (414) for sires A and B, respectively], but it was higher (P < 0.01) for conventional semen [63.7 (422) v. 52.6 (418)]. An interaction between sire and type of semen (P = 0.07) revealed that sex-selected semen from bull B had the lowest P/TAI [49.3 (203)]. Overall, heifers with an ED patch scored 2 had greater (P < 0.01) P/TAI [64.8 (526)] than those scored 0 [42.9 (170)] or 1 [46.8 (79)]. Heifers with a missing ED patch had intermediate P/TAI [58.5 (65)]. However, the effect of ED patch scoring on P/TAI depended on type of semen. In heifers inseminated with conventional semen, PTAI was significantly lower (P < 0.01) in those with an ED patch scored 0 [49.4(87)] compared to those with a patch scored 2 [69.6 (270)]. In heifers inseminated with sex-selected semen, P/TAI was significantly lower (P < 0.05) in those with an ED patch scored 0 [36.1(83)] or 1 [37.2 (43)] compared with those scored 2 [59.8 (256)] or 3 [58.3 (36)]. In summary, P/TAI was greater in heifers inseminated with conventional semen. However, the P/TAI obtained with the modified GnRH-based TAI protocol in cyclic heifers inseminated with sex-selected semen was encouraging. In addition, the Estrotect oestrus detection patches were considered useful to identify animals for TAI with sex-selected semen and could be used to increase the adoption of this technology in beef herds.