The study evaluated the effect of a GnRH treatment at the time of timed-artificial insemination (TAI) on fertility of Bos indicus (Nelore) beef cows. We hypothesized that GnRH treatment at the time of TAI would increase pregnancy per AI (P/AI). Primiparous and multiparous cows from two farms (A: n = 699; B: n = 749), over two breeding seasons (BS), were submitted to a 7-d estradiol (E2)/progesterone (P4)-based TAI protocol. In farm A, cows were enrolled in this study only at the second TAI (resynchronization), whereas cows from farm B were enrolled at the first and second TAI. In both farms, on D0, body condition score (BCS) was evaluated and all cows received a 1 g intravaginal P4 device, 2 mg E2 benzoate (EB), and 0.53 mg cloprostenol sodium (PGF). On D7, concomitant with P4 device removal, cows received 0.53 mg PGF, 1 mg E2 cypionate and 300 IU eCG. On D9, expression of estrus was evaluated, TAI was performed and cows were randomly assigned to receive (G1) or not (G0) a treatment with GnRH at the time of TAI (farm A: 8,4 µg buserelin acetate [G0 = 347 and G1 = 352]; farm B: 25 µg lecirelin [G0 = 364 and G1 = 385]). All products were from GlobalGen, except for lecirelin that was from Agener. Pregnancy diagnosis was performed by ultrasound 30 d after TAI. Statistical analyses were performed by the GLIMMIX procedure of SAS 9.4 (P ≤ 0.05). In farm A, presence of CL on D0 was greater in multiparous than primiparous cows (72.6% [336/463] vs. 39.0% [92/236]; P < 0.0001). Presence of CL on D0 was greater in cows with BCS ≥ 3 (66.9 [238/356] vs. 55.4% [190/343]; P = 0.02), and those cows had greater expression of estrus than cows with BCS < 3 (84.8 [302/356] vs. 73.8% [253/343]; P = 0.002). Moreover, more multiparous expressed estrus than primiparous cows (86.0 [398/463] vs. 66.5% [157/236]; P < 0.0001). Cows with CL on D0 had greater P/AI than cows without CL (65.4 [280/428] vs. 50.6% [137/271]; P = 0.05). Besides, P/AI was greater in cows that expressed estrus in comparison with cows that did not express estrus (64.0% [355/555] vs. 43.1% [62/144]; P = 0.0009). There were no interactions between GnRH treatment at TAI and CL on D0, BCS, or parity. Cows receiving GnRH had greater overall P/AI than cows that did not receive (62.5% [220/352] vs. 56.8% [197/347]; P = 0.05) and GnRH treatment at TAI increased P/AI of cows not expressing estrus (52.1% [37/71] vs. 34.3% [25/73]; P = 0.05). In farm B, GnRH at TAI did not interact with parity, number of AI, BCS, or estrus. Likewise, the GnRH treatment increased P/AI (G1 = 54.3% [209/385] vs. G0 = 49.7% [181/364]; P = 0.02). In conclusion, our hypothesis was supported, and the study reinforced the benefit of a GnRH treatment at TAI on increasing P/AI of Bos indicus (Nelore) beef cows.