173 A COMBINED RECOMBINANT BOVINE SOMATOTROPIN/EQUINE CHORIONIC GONADOTROPIN PROTOCOL IN THE ZEBU BREED TABAPUA AND HOLSTEIN HEIFERS SUBMITTED TO OVUM PICKUP

Abstract

The aim of this study was to enhance ovarian follicular development and subsequent oocyte retrieval by the use of a combined equine chorionic gonadotropin (eCG) and recombinant bovine somatotropin (rbST) protocol, as well as to compare its efficiency between the Brazilian zebu breed Tabapua (TAB) and Holstein (HOL) nulliparous heifers. In a changeover design, TAB (n = 16) and HOL (n = 12) were allocated to 1 of 2 protocols in 2 distinct periods. Two days before initiation of the protocols (D2), dominant follicles were ablated by transvaginal ultrasound-guided aspiration: Group 1: on day 0 (D0) a subcutaneous injection of 500 mg of rbST (Boostin®, Intervet, SP) and on D2 an intramuscular injection of 500 IU of eCG (Novormon®, Schering-Plough-Intervet, SP); and control: vehicle only. Two days later (D4), ovum pickup (OPU) was performed and the ovarian follicle population was evaluated by ultrasound. Twenty days after the first OPU session, heifers were switched over across treatments and were aspirated a second time, so that a total of 28 sessions were performed for each treatment. Main effects of period, breed, and protocol and their interactions on follicle numbers and oocyte yield were analysed through the GENMOD procedure of SAS (SAS^®, Cary, NC, USA) using the Poisson distribution option. Means were compared by orthogonal contrasts, and the probability value set at 0.05 for significance unless otherwise specified. There was an interaction effect between hormonal stimulation and breed (P < 0.05) on the number of aspirated follicles (2 to 10 mm in diameter) such that they were higher in TAB treated (41.4 ± 2.6) compared with control (29.9 ± 2.6) heifers and did not differ between the HOL heifers (15.5 ± 2.6 v. 14.4 ± 2.6 in treated and control heifers, respectively). Overall, follicle numbers were higher (P < 0.0001) in TAB (35.6 ± 1.84) than in HOL (15.0 ± 2.1) heifers and in treated (28.4 ± 2.0) v. control (22.1 ± 2.0) heifers (P < 0.001). There was no effect (P = 0.77) of treatment on total viable oocytes (grades 1 to 3) between treated (3.5 ± 1.2) and control (3.9 ± 1.1) heifers. Tabapuã heifers had more (P < 0.05) viable oocytes than HOL (5.9 ± 0.9 v. 1.6 ± 1.4). In conclusion, the proposed protocol may improve OPU results but viable oocyte yield was not associated to the higher number of follicles available for aspiration. It may be necessary to modify this protocol to improve results perhaps by increasing the time window between rbST and eCG injections as well as by increasing the rbST dosage for heifers. The present protocol may be cost saving, compared with follicle stimulating hormone, for initial OPU sessions in the case of Tabapuã heifers because it is expected that responses decrease after a sequence of eCG injections. It is also interesting to notice that it is possible that HOL heifers already have higher endogenous growth hormone concentrations, which could help to explain the interactive effect observed in the ovarian response. Further studies are necessary to improve this protocol especially in HOL heifers.