Utilização de diferentes doses de Gonadotrofina coriônica equina sobre o crescimento folicular e taxa de prenhez em novilhas de Corte Bos Taurus submetidas a programa de Inseminação Artificial em tempo fixo

Abstract

With the potential for expansion of the Brazilian beef market, investment in technologies that improve the reproductive efficiency of beef cattle is essential for the intensification of production. Biotechniques such as fixed-time artificial insemination (TAI) contribute, accelerating genetic and productive gains and intensifying the use of females of reproductive age. Although it is a consolidated tool, research prioritizes adjustments in hormonal protocols, considering the particularities of genetic groups and animal categories. The administration of gonadotropins, such as equine chorionic gonadotrophin (eCG), is widely studied, aiming to raise conception rates. The objective of the present study was to evaluate the effect on follicular growth and pregnancy rate using different doses of eCG in B. taurus heifers submitted to TAI program. A total of 1080 heifers with 24 and 26 months and body condition score (BCS) of 2.90 ± 0.02 (1-cachectic 5-obese) were used in 8 commercial farms in the state of Rio Grande do Sul, Brazil. On day 0 (D0), heifers were evaluated for cyclicity (CLD0) and BCS and received a slow release device of P4 (CIDR®, Zoetis, Campinas, SP, Brazil) and 2 mg of benzoate of estradiol IM (E2; Gonadiol®, Zoetis, Brazil). On day 7 (D7), 12.5 mg of dinoprost tromethamine IM (PGF; Lutalyse®, Zoetis, Brazil) were given and on day 9 (D9) the P4 device was removed and 0.5 mg estradiol cypionate IM (ECP®, Zoetis, Brazil). At this time, heifers were homogenously divided into 3 treatments: 200 IU (n = 387), 300 IU (n = 357) or 400 IU (n = 336) of eCG. All were painted at the base of the tail with an ink stick (Raidl-Maxi, Raidex GmbH, Dettingen / Erms, Germany) at the time of removal of P4. The animals observed without paint at the time of TAI were considered in estrus. TAI was performed 48 hours after removal of the P4 implant (D11). In addition, 213 females were examined by ultrasonography on days D9 and D11 to measure the diameter of the largest follicle (FL) and day (D18) of the corpus luteum (CL). The pregnancy rate was performed at D41. For the statistical analysis, the GLIMMIX SAS procedure was used, and the variables compared by the Tukey-Kramer test (P <0.05). Reduction of the eCG dose did not influence follicular growth (P = 0.22); the diameter of the largest FL in D11 (P = 0.79); occurrence of estrus (P = 0.92); (P / IA, 52.2% ± 2.54, 49.77% ± 2.63, 51.48% ± 2.73, P = 0.46), and the CL diameter (18.60 ± 0, 57; 17.92 ± 0.67 and 18.93 ± 0.5, P = 0.29), respectively, for dose 200, 300 and 400 IU. However, the presence of CL in the D0 led to larger sizes of the dominant follicle in D9 and D11 (P = 0.006 and P <0.0001), CL diameter in D17 (<0.0001) and P / IA when compared to heifers without CL (57.7% vs. 44.8% P = 0.01). Based on these results, it was concluded that reducing the dose of eCG to 200 IU did not impair fertility of heifers, and the status cycling status before the breeding season proved to be an important factor for predict the success of the application-based TAI programs of E2 and exposure to P4.