PS1-9. Effects of Intravaginal Progesterone Device Size and Dose on Reproductive Performance and Growth of Bos Indicus Heifers.

Abstract The increasing use of heifers in breeding programs requires reproductive technologies adapted to this category, which is characterized by lower body weight and greater hormonal sensitivity compared with mature cows. Therefore, the objective of this study was to evaluate the effects of different intravaginal device sizes and progesterone (P4) doses used in fixed-time artificial insemination (FTAI) protocols on reproductive and growth performance of beef heifers. At the beginning of the experiment, body weight (BW) and body condition score (BCS; 1–5 scale) were recorded, and transrectal ultrasonography was performed to evaluate ovarian structures in eighty-three Nellore (Bos indicus) heifers. On day zero (D0), the heifers were randomly assigned to two treatments: RNeo (n = 42), which received a conventional-sized intravaginal device containing 1.0 g of P4 (Repro Neo®, GlobalGen); and OneN (n = 41), which received a reduced-size intravaginal device containing 0.5 g of P4 (Repro One Novilhas®, GlobalGen). On the same day, all heifers received 2 mg of estradiol benzoate (EB; Syncrogen®, GlobalGen), and those presenting a corpus luteum (CL) received 0.5 mg of prostaglandin F2α (PGF2α; Induscio®, GlobalGen). On day 8 (D8), P4 devices were removed and 300 IU of equine chorionic gonadotropin (eCG; eCGen®, GlobalGen), 0.5 mg of PGF2α, and 1.0 mg of estradiol cypionate (ECP; Cipion®, GlobalGen) were administered, followed by tail-head marking for estrus detection. On day 10 (D10), heifers were weighed again, evaluated for estrus expression based on tail-head paint removal, received 8 μg of gonadotropin-releasing hormone (GnRH; Maxrelin®, GlobalGen), and were subjected to FTAI. Pregnancy diagnosis was performed 31 days after FTAI. At the beginning of the protocol, a tendency (P = 0.06) for greater corpus luteum (CL) presence was observed in the RNeo group compared with OneN (71.41% vs 51.21%). At D0, body weight and body condition score did not differ between groups, averaging 321.32 ± 33.45 kg and 3.14 ± 0.36 in OneN heifers and 327.40 ± 29.40 kg and 3.28 ± 0.46 in RNeo heifers. However, by D10 a lower proportion of heifers gained weight in the OneN group (58.53%) compared with RNeo (78.57%; P = 0.05).All heifers in both groups expressed estrus, and conception rate did not differ between treatments, averaging 48.78% in OneN and 50.00% in RNeo (P ≥ 0.99). Therefore, the P4 device did not affect mean body weight, estrus expression, or conception rate; however, a greater proportion of heifers gained weight in the conventional device group. These findings suggest a possible effect of P4 dose on short-term growth responses in Nellore heifers.