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The primary objective of this study was to investigate the impact of animal-level factors including energy balance and environmental/management stress, on the ovarian function of Bos indicus heifers treated to synchronize ovulation. Two-year-old Brahman (BN) (n = 30) and BN-cross (n = 34) heifers were randomly allocated to three intravaginal progesterone-releasing device (IPRD) treatment groups: (i) standard-dose IPRD [Cue-Mate(®) (CM) 1.56 g; n = 17]; (ii) half-dose IPRD [0.78 g progesterone (P(4)); CM 0.78 g; n = 15]; (iii) half-dose IPRD + 300 IU equine chorionic gonadotrophin at IPRD removal (CM 0.78 g + G; n = 14); (iv) and a control group, 2× PGF(2α) [500 μg prostaglandin F(2α) (PGF(2α))] on Day -16 and -2 (n = 18). Intravaginal progesterone-releasing device-treated heifers received 250 μg PGF(2α) at IPRD insertion (Day -10) and IPRD removal (Day -2) and 1 mg oestradiol benzoate on Day -10 and -1. Heifers were managed in a small feedlot and fed a defined ration. Ovarian function was evaluated by ultrasonography and plasma P(4) throughout the synchronized and return cycles. Energy balance was evaluated using plasma insulin-like growth factor 1 (IGF-I) and glucose concentrations. The impact of environmental stressors was evaluated using plasma cortisol concentration. Heifers that had normal ovarian function had significantly higher IGF-I concentrations at commencement of the experiment (p = 0.008) and significantly higher plasma glucose concentrations at Day -2 (p = 0.040) and Day 4 (p = 0.043), than heifers with abnormal ovarian function. There was no difference between the mean pre-ovulatory cortisol concentrations of heifers that ovulated or did not ovulate. However, heifers that ovulated had higher cortisol concentrations at Day 4 (p = 0.056) and 6 (p = 0.026) after ovulation than heifers that did not ovulate.

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Estrous synchronization (ES) protocols for fixed-time AI (FTAI) have been mainly developed for Bos taurus cattle assuming that usage in Bos indicus cattle results in similar outcomes. Unexplained variation in the reproductive outcomes of these treatments in extensively managed Bos indicus herds has limited the adoption of FTAI. The aim was to evaluate the ovarian function in heifers treated with different doses of progesterone (P4). Brahman (n = 30) and Brahman-cross (n = 34; 3/4 to 7/8 Brahman) heifers weighing an average of 285 kg (248 to 323 kg) and 307 kg (250 to 363 kg), respectively, were allocated at random to 1 of 4 treatment groups: 1) full-dose Cue-Mate (CM100; two P4-impregnated pods, 1.56 g of P4; Bioniche Animal Health Aust/Asia, Armidale, NSW, Australia; n = 17); 2) half-dose Cue-Mate (CM50; one P4-impregnated pod; 0.78 g of P4; n = 15); 3) half-dose Cue-Mate + 300IU of eCG (CM50+G; Pregnecol, Bioniche Animal Health, Aust/Asia; n = 14) i.m. at Cue-Mate removal; and 4) double PGF2α group (PG; n = 18). Heifers in the P4-treated groups received 1 mg of EB (estradiol benzoate; Ciderol; 1 mg mL-1; Genetics Australia, Bacchus Marsh, Victoria) at Cue-Mate insertion (Day -10) and 24 h after Cue-Mate removal (Day -1) and 250 μg of PGF2α (Cloprostenol, Estromil, Ilium Veterinary Products, Smithfield, Australia) i.m. at Cue-Mate insertion (Day -10) and removal (Day -2). Heifers in the PG group received 500 μg of PGF2α on Day -16 and Day -2 i.m. Ovarian function was evaluated twice daily by transrectal ultrasonography on Day -1, Day 0 (expected estrus), and Day 1, and once daily on Days 4, 6, 8, 10, 12, and 16 of the synchronized and return cycles. Concurrent blood samples were collected to determine plasma P4 concentrations. A one-way ANOVA was used to analyze follicle diameter variables and Fisher’s Exact test was used to analyze all other data reported (StataCorp 2007, College Station, TX, USA). There was no difference (P = 0.31) in the diameter of the dominant follicle 54 to 56 h post-Cue-Mate removal (the time FTAI would be conducted) in the CM100 (11.5 ± 1.1), CM50 (13.3 ± 1.5), CM50+G (13.7 ± 1.5), or PG (14.5 ± 0.7) groups. Synchrony of ovulation in ovulating heifers did not differ (P < 0.05) between the P4-treated groups with 12/13 (CM100), 11/12 (CM50), and 9/12 (CM50+G) heifers ovulating within 53 to 77 h after Cue-Mate removal, but was lower (P < 0.02) for the PG group (1/5). Individual heifer P4 profiles revealed that 2/15, 1/15, 3/12, and 0/5 of heifers that ovulated in the CM100, CM50, CM50+G, andPG groups, respectively, failed to re-ovulate in the return cycle. Also, 1/15, 3/15, 1/12, and 0/5 of heifers that ovulated in the CM100, CM50, CM50+G, and PG groups, respectively, developed a CL that persisted into the return cycle. This study shows the potential benefits of using intravaginal devices containing a lower amount of P4 to induce synchronized ovulation in Brahman heifers, consistent with the findings of Carvalho et al. (2008 Theriogenology 69, 167).