Effect of different doses of estradiol benzoate in association with constant dose of progesterone on ovarian Structure of Holstein heifers

The objective of this study was to evaluate the effect of different doses of estradiolbenzoate in the presence of constant dose of progesterone injected at early luteal phase, on ovarian structure of Holstein heifers. Estrous cycle of cyclic Holstein heifers weresynchronized using CIDR, for 14 days, in association with GnRH and PGF2α analogues onDay 6 and 13 after CIDR insertion, respectively. On day 5-7 of the ensuing cycle (Day 0 ofexperiment), heifers were assigned into 4 experimental groups. Heifers in groups 1, 2 and 3 received respective dose of 2, 5, 10 mg estradiol benzoate concurrent with 100 mgprogesterone intramuscularly. Heifers in control group received distilled water intramuscularly. Ultrasound examination of the ovary and blood sampling were conducteddaily to detect plasma progesterone concentrations, from Day 0 to 16. On Day 0, there was a functional CL and a growing dominant follicle on the ovary of all heifers. Plasmaprogesterone concentrations were not showed significant differences among treated groups on the day of steroid injection. After injection of steroids, plasma progesterone concentrations raised significantly in the steroid treated groups compared with control group (P<0.05). In treatment groups (2, 5 and 10 mg), the interval from steroid injection to dominant follicle regression was shorter than that of control group (P<0.05). Interval from steroid injection to the decline in plasma progesterone concentration to basal level was shorter in 5 mg (7.7 ± 1.10 day) compared with control group (13.3 ± 1.20 day; P<0.01). In conclusion, estradiol benzoate injected in early luteal phase, may induce corpus luteum regression in a dose dependent manner and ovarian follicle regression independent of the doses (2, 5 and 10 mg), in Holstein heifers.