The effect of steroids and GnRH, given on the day of estrus, on preovulatory dominant follicle (PDF) and synchronization of follicular wave emergence in Holstein heifers was investigated. The stage of estrous cycle was synchronized in cyclic Hostein heifers using two I.M. injection of prostaglandin F2a analogue (PG) 11 days apart. On the day of estrus (Day 0 of experiment) heifers were divided into three groups (control, GnRH and steroid) considering their weight and age. Heifers in each group were kept in the separate barn and received the same ration. Heifers in control group (n4) received no treatment. Within 3 hours of detecting standing estrus. heifers in GnRH (n9) and steroid (n=9) groups were given an I.M injection of GnRH (250 microgram Fertagyl) and combination of estradiol benzoate (2 mg) and progesterone (100 mg), respectively. GnRH and steroid heifers received PG on Day 7 of treatment and estrus detection was simultaneously initiated. From Day —3 to 25 of treatment, ultrasonography was conducted to characterize ovarian follicle. On the day of estrus, PDF was detected in all heifers and ovulated at the same in experimental groups (P>0.05). The first follicalar wave was emerged in GnRH (2.1±0.41 d) and control (2.4±0.18 d) earlier than steroid (3.9±0.23 d) groups (P<0.05). The interval to estrus and ovulation after initiation of treatment was similar (P>0.05) in GnRH (9.2±0.28 d, 10.2±0.28 d) and steroid (9.7±0.71 d, 11±0.64 d) groups compared to control group (22.8±1 . 11 d, 24.3±0.95 d. P<0.0l). The growth rate of ovulatory follicle was similar in GnRH (1.3±0.06 mmld) and steroid (1.4±0.03 mm/d) groups but greater than (P<0.01) control group (0.9±0.06 mm/d). The age of ovulatory follicle in treated groups (GnRH: 5.2±0.22 d; steroid: 4.3±0.64 d; P<0.05) was shorter (P<0.05) than control group (7.3±0.48 d). The tightness of estrus synchrony between 24-72 hours after PG injection was similar between GnRH (77.7%) and steroid (88.8%) groups (P>0.05). The result of this study suggests that GnRH and steroid, given on Day of estrus, have no deliterious effect on ovulation and the new follicular wave emerges at the same time following these treatments. Ovulation of newly recruited dominant follicle occurs after PG injection prescribed 7 days after GnR1-1 or steroid treatment. Therefore, these protocols can he used. alternatively, in estrous synchronization programs in dairy heifers.
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