Hormonal combinations aiming to improve reproductive outcomes of Bos indicus cows submitted to estradiol/progesterone-based timed AI protocols

Abstract

The aim was to study reproductive outcomes of Nelore (Bos indicus) cows submitted to a 7-d estradiol (E2)/progesterone (P4)-based timed artificial insemination (TAI) protocol, receiving various combinations of doses and hormones. Primiparous (n = 962) and multiparous (n = 1935) cows were submitted to synchronization (n = 2012) and resynchronization (n = 885 non-pregnant cows at pregnancy diagnosis 30 d after TAI) protocols, following a 2 × 2 × 2 factorial arrangement of eight treatments. At the initiation of the TAI protocol (Day -9), all cows received a 1.0 g intravaginal P4 insert, 2.0 mg E2 benzoate and received (PGF1) or not (PGF0) 0.5 mg cloprostenol sodium (PGF). On Day -2, the P4 insert was removed, all cows received 0.5 mg PGF, 300 IU equine chorionic gonadotropin (eCG) and 0.5 (EC0.5) or 1.0 mg estradiol cypionate (EC1.0). On Day 0, cows were treated (G1) with 8.4 μg buserelin acetate (GnRH) or not (G0), concurrently with TAI. The eight treatments were generated: 1) PGF0-EC0.5-G0 (n = 364), 2) PGF0-EC0.5-G1 (n = 363), 3) PGF1-EC0.5-G0 (n = 363), 4) PGF1-EC0.5-G1 (n = 360), 5) PGF0-EC-1.0-G0 (n = 360), 6) PGF0-EC1.0-G1 (n = 363), 7) PGF1-EC1.0-G0 (n = 361), and 8) PGF1-EC1.0-G1 (n = 363). Pregnancy per AI (P/AI) was greater at first AI compared with resynchronization (58.9 [n = 2012] vs. 54.9% [n = 885]). Presence of CL on Day -9 resulted in more cows expressing estrus (81.3 [n = 680] vs. 67.1% [n = 2033]) and greater P/AI (66.0 [n = 692] vs. 54.9% [n = 2106]). There was no difference in P/AI between cows that received or not PGF on Day -9 (58.7 [n = 1447] vs. 56.6% [n = 1450]). In contrast, PGF tended to increase P/AI of cows with CL on Day -9 (with PGF = 69.1 [n = 375] vs. without PGF = 62.5% [n = 317]). Cows that received 1.0 mg EC expressed more estrus than those treated with 0.5 mg (73.8 [n = 1414] vs. 67.9% [n = 1398]) and had greater P/AI (60.2 [n = 1447] vs. 55.1% [n = 1450]). P/AI was greater in cows treated with GnRH at TAI (59.8 [n = 1449] vs. 55.5% [n = 1448]), particularly in cows that did not show estrus (52.7 [n = 393] vs. 38.1% [n = 420]). Moreover, GnRH on Day 0 increased P/AI in cows with BCS < 3.0 (57.1 [n = 723] vs. 48.6% [n = 698]), in primiparous (50.1 [n = 465] vs. 41.9% [n = 497]) and in cows that received 0.5 mg EC (58.9 [n = 723] vs. 51.3% [n = 727]). In conclusion, 1.0 mg of EC on Day -2 and GnRH at TAI improved P/AI, but the combination of a higher dose of EC and GnRH treatment at AI did not enhance this effect. Furthermore, GnRH improved P/AI especially in Bos indicus cows with lower expression of estrus, such as primiparous, thinner cows, and cows treated with 0.5 mg of EC.

Keywords: Beef cattle; Estrus; Fertility; GnRH; Synchronization; TAI.