211 EFFECT OF ESTRADIOL BENZOATE ON THE FIRST WAVE DOMINANT FOLLICLE IN HOLSTEIN HEIFERS

The objective of this study was to evaluate the effect of different doses of estradiol benzoate on the first wave dominant follicle in Holstein heifers. Estrous cycle of cyclic Holstein heifers (n = 16; 13.84 ± 0.44 months of age; 335.5 ± 6.65 kg) was synchronized using CIDR (14 days; VetoCIDR®, Aburaihan, Tehran, Iran) in association with GnRH analogue (12.5 µgi.m. Vetaroline®, Aburaihan) on Day 6 and PGF2 analogue (500 µg; Vetaprost®, Aburaihan) on day 13 after CIDR insertion. On Days 5 to 7 of the ensuing cycle (Day 0 of the experiment), heifers were assigned into 4 experimental groups. Heifers in Groups 1, 2, and 3 (n = 4 in each group) received 2, 5, and 10 mg of estradiol benzoate (i.m. Vetastrol®, Aburaihan), respectively, concurrent with 100 mg of progesterone (i.m. Vetagesterone®, Aburaihan). Heifers in the control group (n = 4) received distilled water as a placebo intramuscularly. Ovarian follicles were monitored daily, from Days 0 to 16, using an ultrasound scanner (Sonovet 2000, Korea) equipped with a 7.5-MHz transrectal probe. Initiation of follicle regression was defined when there was a sudden decrease of 1 to 2 mm followed by a progressive decrease in the diameter of the ovarian follicle. Emergence of anew follicle wave was defined as the day that the dominant follicle was, retrospectively, identified at a diameter of 4 to 5 mm. Data were analyzed using Kruskal-Wallis one-way ANOVA in SAS (SAS Institute, Cary, NC). On Day 0, there was a functional CL (19.62 ± 0.57 mm) and a growing dominant follicle (11.25 ± 0.62 mm) on the ovary of all heifers. The dominant follicle regressed on Day 3.5 ± 1.04, 3.2 ± 0.85, 2.7 ± 0.75, and 8.5 ± 0.50 in Groups 1, 2, 3, and control, respectively (P < 0.05). A new follicle wave emerged on Day 7.7 ± 0.25, 8.2 ± 0.48, 9.7 ± 1.44, and 7.2 ± 1.11 in Groups 1, 2, 3, and control, respectively (P > 0.05). Estrus was detected on Day 12.0 ± 1, 11.7 ± 0.34, 13.0 ± 0.00, and 15.2 ± 1.75 in Groups 1, 2, 3, and control, respectively (P > 0.05). The interval between steroid injections (Day 0) and ovulation were 13 ± 0.41, 13 ± 0, 14.3 ± 0.33, and 16.2 ± 1.75 days for Groups 1, 2, 3, and control, respectively (P > 0.05). Interovulatory intervals were 18.7 ± 0.33, 18.7 ± 0.33, 20.7 ± 0.67, and 20.5 ± 1.55 days for Groups 1, 2, 3, and control, respectively (P > 0.05). In conclusion, estradiol benzoate induced early regression of the dominant follicle of the first wave but did not affect the interval between steroid treatment and ovulation in Holstein heifers. This research was funded by Aburaihan Pharmaceutical Company (Tehran, Iran).