8 Evaluation of puberty induction protocol in peripubertal beef heifers prior to fixed-time AI programs

Beef heifers that conceive early during their first breeding season calve earlier, wean heavier calves, and tend to continue doing so throughout the rest of their reproductive lives. Otherwise, under extensively managed systems in Southern Brazil, only a variable proportion of 2-year-old beef heifers reach puberty before the start of the breeding season. The aim of this study was to evaluate a protocol to induce puberty in peripubertal beef heifers using injectable progesterone (P4) and oestradiol cypionate (EC) to improve pregnancy per AI (P/AI) in fixed-time AI (FTAI) programs. A total of 311 Angus heifers, 24 ± 3 months of age, 3.1 ± 0.3 body condition score (1 = emaciated; 5 = obese), and 328 ± 41 kg of body weight (BW), were examined before FTAI protocol (Day −24) by transrectal ultrasonography to determine reproductive tract score (RTS; 1 = prepubertal; 5 = pubertal). Prepubertal heifers (RTS 1; 12.8%, 40/311) were excluded from the study. Heifers with RTS = 2 (n = 271) were assigned to 2 groups: P4 (RTS 2 or 3, n = 100) and cyclic (RTS 4 and 5, n = 171). Peripubertal heifers were treated on Day −24 with 150 mg of injectable P4 IM (Sincrogest® injectable, Ourofino Animal Health, Cravinhos, Brazil) and on Day −12 received 1 mg of EC IM (Sincro CP®, Ourofino Animal Health). Cyclic heifers received no hormonal treatment before FTAI protocol. On Day 0, heifers in the P4 group were evaluated again to determine RTS, body condition score, and BW. Induction of puberty was defined by the presence of a follicle >8.5 mm (RTS = 4) or a corpus luteum (RTS = 5). All heifers received an FTAI protocol consisting of 2 mg of oestradiol benzoate IM (Sincrodiol®, Ourofino Animal Health) and an intravaginal progesterone implant (1 g; Sincrogest®, Ourofino Animal Health). On Day 8, heifers had implants removed and were injected with 1 mg of EC and 500 µg of sodium cloprostenol IM (Sincrocio®, Ourofino Animal Health). After 48 h, all heifers received an FTAI. Thirty days after AI, the diagnosis of gestation was performed by ultrasonography. Statistics included chi-square and linear regression with coefficients analysed as independent variables, considering a significant P-value of 0.05. On Day 0, percentages of RTS 5, 4, and 3 in the P4 heifers were 37, 27, and 36%, respectively, resulting in 64% of puberty induction. The P/AI was similar (P = 0.14) in P4-treated and cyclic heifers (59 v. 50.3%, respectively). The regression analysis presented a linear and positive effect of BW on P/AI (P < 0.001) in cyclic heifers and a positive influence of BW on RTS (P = 0.05) and RTS on P/AI (P = 0.008) in P4-treated heifers. The application of RTS system associated with P4 treatment of peripubertal heifers before FTAI protocol improved ovarian activity and might have contributed to higher P/AI, comparable with that achieved in cyclic heifers.