الفهرس 
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Abstract
Poor estrus expression and poor estrus detection efficiency in dairy buffalo impaired buffaloes’ reproductive usefulness. Successful synchronization of ovulation in the conventional Ovsynch protocol depends on the ovulatory response to the first gonadotropin-releasing hormone (GnRH injection). The objective of the present study was to evaluate the administration of an additional dose of GnRH or human chorionic gonadotropin (hCG) 24 h after the first GnRH (G1) of Ovsynch protocol on ovulatory response, follicular and luteal dynamics and hemodynamics, serum progesterone (P4) concentrations, synchronization, and pregnancy rates in Egyptian buffaloes (Bubalus bubalis) during the breeding season (Winter and Spring). For this purpose, two experiments were done. The first experiment was done on fifty (n=50) multiparous Egyptian buffalo cows of an unknown stage of the estrous cycle. Animals were divided into two groups (each one including 25 buffalo cows), Ovsynch and G6G-Ovsynch groups. The second trial was done on a total of sixty (n=60) multiparous Egyptian buffalo cows. They were randomly assigned into four groups, including Ovsynch (n=15), G6G-Ovsynch (n=15), Ovsynch-GnRH 24 (n=15), and Ovsynch-hCG 24 (n=15) groups. Buffaloes were subjected to timed artificial insemination (TAI) 16 h and 40 h after the last GnRH (G2) injection in all groups. Blood sampling and ovarian transrectal ultrasonography were conducted at three time points: day 7, the time of G2, and two days after G2. All results were expressed as Mean± SEM and p ˂ 0.05 was considered statistically significant. Data were analyzed using the Chi-square test to compare the binomial variables and general linear model for repeated measures for contentious variables. The ovulatory response at the beginning of protocol and serum P4 (ng/ml) concentrations were significantly increased in the G6G-Ovsynch, Ovsynch-GnRH 24, and Ovsynch-hCG 24 groups at the time of PGF2α injection. The luteal diameter (mm), luteal area (mm)2, and luteal volume (mm)3 were significantly increased in the Ovsynch-GnRH 24 and Ovsynch-hCG 24 groups. A larger (P ˂ 0.05) total colored area (mm)2 of corpus luteum (CL) was observed in the G6G-Ovsynch, Ovsynch-GnRH 24, and Ovsynch-hCG 24 groups. The luteal echogenicity and vascularity index (%) were higher (P ˂ 0.05) in the G6G-Ovsynch group. A higher (P ˂ 0.05) vascularity index (%) of the dominant follicle (DF) wall was observed in the G6G-Ovsynch, Ovsynch-GnRH 24, and Ovsynch-hCG 24 groups. The synchronization rate (%) significantly increased in the G6G-Ovsynch, Ovsynch-GnRH 24, and Ovsynch-hCG 24 groups. The pregnancy rate was higher in the G6G-Ovsynch, Ovsynch-GnRH 24, and Ovsynch-hCG 24 groups compared to the Ovsynch group. In conclusion, the administration of an additional dose of GnRH or hCG 24 h after the first GnRH of the Ovsynch protocol improved the ovulatory response at the beginning of the Ovsynch protocol, subsequently leading to increased serum P4 concentrations on day 7, synchronization rate (%), and pregnancy rates among Egyptian buffaloes.