The effectiveness of low-dose of eCG in timed AI Nelore (Bos indicus) heifers.

This study evaluated the efficacy of the administration of different doses of equine chorionic gonadotropin (eCG; 0 IU, 200 IU, or 300 IU) at the time of the progesterone device removal in 2-year-old Nelore (Bos indicus) heifers synchronized for fixed-timed artificial insemination (FTAI). On day 0 (D0), a total of 398 heifers received 2 mg of oestradiol benzoate i.m., 0.53 mg of cloprostenol i.m., and an eight-day previously used (second use) intravaginal device containing 1 g of progesterone (P4). Eight days later (D8), simultaneous with the P4 device removal, 0.5 mg of oestradiol cypionate i.m. and 0.53 mg of cloprostenol i.m. were administered. At the same time, heifers were randomly assigned to receive one of the following treatments: G-0 IU (n = 141; no eCG treatment), G-200 IU (n = 132; treated with 200 IU of eCG), and G-300 IU (n = 125; treated with 300 IU of eCG). FTAI was performed 48 h after the P4 device removal (D10). Ultrasonographic evaluations were performed at D0, D10, and D17. Heifers were scanned to measure the size of the largest follicle (LF), the presence, number, and size of the corpus luteum (CL), and the ovulation rate. Subsequently, at D40, the heifers underwent scanning to determine the pregnancy rate and identify any twin pregnancies. Additionally, at D70, scans were performed to assess pregnancy loss (PG). Data were analysed by orthogonal contrasts [C1 (eCG effect): control x (200 IU + 300 IU) and C2 (eCG dose effect): 200 IU × 300 IU]. On D0, CL presence was similar between the groups [G-0 IU = 65.2% (92/141), G-200 IU = 55.3% (73/132), and G-300 IU = 63.2% (79/125); p = .16]. No interactions between the presence of CL on D0 and eCG treatment were found for any of the variables (p > .05). The diameter of the LF at FTAI (D10) was not influenced by eCG treatment (p = .22) or eCG dose (p = .18). However, treatment with eCG increased the diameter of the CL at D17 (G-0 IU = 15.7 ± 0.3 mmb , G-200 IU = 16.6 ± 0.2 mma , and G-300 IU = 16.6 ± 0.3 mma ; p = .001), regardless of the dose used (p = .94). The ovulation rate was higher in heifers treated with eCG [G-0 IU = 79.4%b (112/141), G-200 IU = 90.2%a (119/132), and G-300 IU = 93.6%a (117/125); p = .002], but there was no effect of eCG dose (p = .36). Pregnancy per AI (P/AI) on D40 [G-0 IU = 32.6%b (46/141), G-200 IU = 42.4%a (56/132), and G-300 IU = 42.4%a (53/125); P = 0.05] and D70 [G-0 IU = 29.1%b (41/141), G-200 IU = 40.9%a (54/132), and G-300 IU = 40.8%a (51/125); p = .02] were higher on heifers that received eCG; however, no dose effect was observed for P/AI on D40 (p = .89) nor D70 (p = .98). Pregnancy loss between D40 and D70 tended to reduce (p = .07) in eCG-treated heifers without dose effect (p = .91). Heifers with CL at D0 presented a greater follicle diameter (LF) on D10 (With CL = 11.2 ± 0.2 mm and Without CL = 10.2 ± 0.2 mm; p = .05), CL diameter on D17 (With CL = 15.8 ± 0.03 mm and Without CL = 11.8 ± 0.6 mm; p = .01), and ovulation rate [With CL = 95.5% (233/244) and Without CL = 74.7% (115/154); p = .01]. However, no difference in pregnancy rate at D40 (p = .52) and D70 (p = .84) was found. In conclusion, eCG treatment increases ovulation and pregnancy rates of heifers submitted to a FTAI protocol. Furthermore, eCG treatment increases the diameter of the CL after FTAI and reduces pregnancy losses. No dose effect was observed, suggesting Nelore (Bos indicus) heifers respond to 200 IU of eCG treatment for FTAI.

Synchronization treatments previous to natural breeding anticipate and improve the pregnancy rate of postpartum primiparous beef cows.

Timed AI has become a potential tool to bypass postpartum acyclicity, yet only a small percentage of the world bovine herd is inseminated. Most females are still subjected to bull mating; therefore, the frequent occurrence of postpartum anestrus may compromise their reproductive efficiency. Thus, the aim of this study was to develop an approach that allows the early conception of postpartum primiparous beef cows that are exposed to natural breeding (NB). For this purpose, 350 primiparous Nelore cows 35-65 d postpartum were allocated into three groups: Control (no hormonal treatment), TNB (hormonal protocol for timed-NB without equine chorionic gonadotropin; eCG) and TNB + eCG (hormonal protocol for TNB with eCG) groups. The protocol for TNB consisted of the insertion of a 1-g progesterone device and the intramuscular (IM) administration of 2 mg estradiol benzoate on D-9 (nine days before bull exposure), followed by device removal and the administration of 1 mg estradiol cypionate IM on D0. An additional 300 IU of eCG was given only to TNB + eCG cows. All cows were exposed to bull mating from D0 to D105. Pregnancy was checked by ultrasonography every 50 d, gestational age was estimated, and the number of new gestations every 21-d cycle (P21, P42, P63, P84 and P105) was predicted. Control cows had lower pregnancy at P21 (5.7%c, 7/123) than TNB (30.4%b, 35/115) and TNB + eCG (51.8%a, 58/112; P = 0.001) cows. Pregnancy rate increased across P42, P63 and P84 (P = 0.001) but remained higher for TNB + eCG cows. Regarding time to conception, the TNB + eCG cows achieved a greater than 50% pregnancy rate within the first days after bull exposure, while the TNB and Control cows took more than 40 d and 90 d to achieve 50% pregnancy rates, respectively. At the end of the breeding season (BS), TNB + eCG cows had a 21% higher pregnancy rate than the Control cows and a 16% higher rate than the TNB cows. The probability of conceiving increased 1.5-fold for cows treated with TNB (P = 0.0079) and 2.2-fold for cows additionally treated with eCG (P < 0.0001). The average interval between the onset of the BS and conception was reduced (P < 0.0001) for the TNB + eCG cows (26.5 ± 3.8c) compared to the TNB (35.7 ±â€¯4.1b) and Control (64.7 ±â€¯3.9a days) cows. Thus, the use of TNB, especially when associated with eCG, efficiently improved the early conception of postpartum primiparous beef cows after exposure to NB. The increased number of cows conceiving early in the BS is crucial to improve reproductive efficiency by reducing the interval between parturitions and improving the number of pregnant cows at the end of BS, thus resulting in greater farm income.