Reproductive performance of yearling Bos indicus heifers submitted to programs for induction of ovulation prior to a timed-artificial insemination protocol.

The aim of this study was to evaluate the effects of two strategies for induction of ovulation prior to the first timed-artificial insemination (TAI) protocol on reproductive outcomes of yearling Nelore (Bos indicus) heifers. On Day -24, a total of 2000 heifers (12.3 ± 0.03 mo old; 224.9 ± 0.7 kg of body weight [BW]), managed in a feedlot system, were randomly assigned to one of two ovulation induction protocols: 1) P4+E2 (n = 963): an intravaginal progesterone (P4) device (0.5 g) was inserted on Day -24 and removed on Day -12, when heifers received 0.5 mg of estradiol (E2) cypionate (EC); 2) iP4 (n = 1037): administration of 150 mg of long-acting injectable P4 on Day -24. All heifers received the same TAI protocol that started on Day 0 with the insertion of a P4 device (0.5 g) and 1.5 mg of E2 benzoate. On Day 7, 0.5 mg of cloprostenol sodium (PGF), 0.5 mg of EC and 200 IU of equine chorionic gonadotropin (eCG) were administered, concomitant with the P4 device withdrawal and tail chalk for estrus evaluation. On Day 9, 8.4 μg of buserelin acetate was administered, and TAI was performed. Ultrasound examinations were performed to evaluate uterine (UT) diameter and ovarian dynamics, pregnancy per AI (P/AI) and pregnancy loss (PL). Statistical analyses were performed by the PROC GLIMMIX of SAS 9.4 (P ≤ 0.05). The P4+E2 treatment resulted in greater incidence of corpus luteum (CL) on Day 0 (57.8 vs. 15.7 %), greater expression of estrus near TAI (66.7 vs. 57.8 %), and greater P/AI on Days 40 and 120 (48.4 vs. 42.1 % and 43.2 vs. 36.3 %, respectively) than the iP4 treatment. However, the incidence of UT > 10 mm (86.7 vs. 85.4 %, respectively), the presence of CL on Day 40 in nonpregnant heifers (20.1 vs. 22.0 %, respectively) and PL (10.7 vs. 14.0 %, respectively) did not differ between groups. Regardless of treatment, the presence of CL on Day 0 was associated with greater expression of estrus (69.0 vs. 58.3 %) and P/AI on Days 40 and 120 (56.1 vs. 39.0 %; 50.3 vs. 33.6 %, respectively), as well as heifers expressing estrus had greater P/AI. Younger heifers had lower incidence of CL on Day 0 than intermediate age and older heifers. However, within all age classes, the P4+E2 treatment resulted in more heifers with CL on Day 0. The P/AI on Day 120 was greater in intermediate and older heifers, whereas younger heifers had greater PL. In addition, heifers with intermediate and heavier BW, as well as those with intermediate and higher average daily gain (ADG), had greater incidence of CL on Day 0, expression of estrus and P/AI on Days 40 and 120. In conclusion, the addition of E2 as an ovulation inducer at the end of an intravaginal P4 supplementation enhanced ovulation induction on Day 0, expression of estrus, and P/AI on Days 40 and 120 in yearling Nelore heifers compared to iP4. Age, BW, and ADG positively influenced the presence of CL, with BW and ADG significantly improving fertility outcomes. These findings emphasize the importance of nutritional management to accelerate body development and optimize reproductive performance in young heifers.