Reproductive outcomes of prepubertal Bos indicus beef heifers raised in a feedlot system submitted or not to ovulation induction strategies prior to a timed-artificial insemination treatment regimen

Reproductive outcomes were evaluated in prepubertal Nelore (Bos indicus) heifers submitted or not to two ovulation induction treatment regimens based on progesterone (P4) and estradiol treatments prior to the timed-artificial insemination (TAI). A total of 583 heifers (11.3 ± 1.3 mo old and 272.2 ± 1.6 kg of body weight [BW]) were randomly assigned to one of two treatments: 0IND (n = 284): no treatment; or 2IND (n = 299): two ovulation induction treatment regimens. On Day −47, heifers from 2IND received an intravaginal P4 device (0.5 g), kept until Day −40, when there was treatment with 0.5 mg of estradiol cypionate (EC). On Day −19, in heifers from 2IND, the same treatment regimen was imposed. On Day 0, all heifers were submitted to the same TAI treatment regimen, starting with a P4 device (0.5 g), 0.53 mg of cloprostenol sodium (PGF_2α), and 1.5 mg of estradiol benzoate. On Day 7, the P4 device was removed, and 0.53 mg of PGF_2α, 0.5 mg of EC, and 200 IU of equine chorionic gonadotropin (eCG) were administered. The TAI was conducted 2 d later (Day 9, 46–50 h after P4 device withdrawal), concurrently with the administration of 8.4 μg of buserelin acetate. Ultrasound evaluations were performed on Days −47, −19 and 0 to evaluate uterine (UT) and largest follicle (LF) diameters and the presence of CL; on Day 40 to detect the presence of CL in nonpregnant heifers; and on Days 40, 70, and 150 (final pregnancy per AI [P/AI]) to assess P/AI and pregnancy losses (PL). Statistical analyses were conducted using SAS 9.4 (^a-cP ≤ 0.05; ^A,B0.05 < P ≤ 0.10). A greater proportion of 2IND-treated heifers had CL on Days −19 (73.9 and 18.3%) and 0 (80.3 and 32.4%) compared to heifers of the 0IND group. On Day 0, the LF diameter was larger in heifers of the 0IND group than 2IND-treated heifers (9.9 ± 0.1 and 8.7 ± 0.1 mm). However, the UT diameter was larger in 2IND-treated heifers on both Days −19 (14.2 ± 0.1 and 13.4 ± 0.1 mm) and 0 (14.0 ± 0.1 and 13.7 ± 0.1 mm). The 2IND-treated heifers had greater expression of behavioral estrus near TAI compared to heifers of the 0IND group (88.0 and 77.8%), although there was no difference in P/AI on Days 40 (39.5 and 41.6%), 70 (33.1 and 37.7%) and final P/AI (32.1 and 35.9%), nor on total PL (18.6 and 13.6%), considering 2IND-treated and heifers of the 0IND group, respectively. Moreover, among heifers without CL on Day 0, 2IND-treated heifers were less fertile than heifers of the 0IND group (final P/AI: 20.3 and 33.9%). Regardless of treatment, presence of CL on Day 0 and expression of behavioral estrus were positively associated with P/AI. Regardless of treatment, BW class (measured on Day 0) positively affected LF and UT diameters, and presence of CL on Days −19 and 0. Moreover, age class positively affected LF and UT diameters, and presence of CL on Days −47, −19, 0 and 40. Age class had no effect on fertility, but BW class was positively associated with final P/AI (heavier: 39.5%^A; intermediate: 33.3%^AB; lighter: 29.8%^B). In conclusion, although the two ovulation induction treatments imposed prior to the synchronization for TAI resulted in a larger proportion of heifers with CL on Day 0, enhanced UT development and improved expression of estrus, there was no improvement in fertility outcomes of prepubertal Nelore heifers managed in a feedlot system. Interestingly, heifers that did not respond to the ovulation induction treatments (without CL on Day 0) were least fertile, indicating that these heifers were not in a peri-pubertal state, rather, the reproductive endocrine status of these heifers was not matured to the extent puberty could be induced with the treatments imposed.