Effect of prostaglandin F2α at the beginning of the ovulation synchronization protocol in dairy bos indicus, bos taurus and bos indicus x bos taurus heifers

Abstract

The objective of the present study was to evaluate the effect of treatment with prostaglandin F2α (PGF2α) at the beginning of the protocol for ovulation synchronization on follicular dynamics in Bos indicus (Gyr; n=11), Bos taurus (Holstein Black and White, HBW; n=10), and crossbred animals (Gyr x HBW; n=12). On a random day of the estrous cycles (day 0, D0), the animals received 2.0 mg estradiol benzoate (EB) intramuscularly plus an intravaginal progesterone (P4) device, which was maintained for 8 days (day 8, D8). Half the heifers of each group received a dose of 25 mg PGF2α intramuscularly at the time of insertion of the intravaginal P4 device. When the intravaginal device was removed (D8), all animals received another dose of 25 mg PGF2α intramuscularly, followed by intramuscular injection of 1.0 mg EB 24 h later (day 9, D9). Ultrasonographic evaluations were performed at intervals of 24 hours from D0 to D8 and at intervals of 12 hours from removal of the P4 device to 96 hours thereafter. Samples were collected on days 0, 3, 6, 8, 10 and 22 for the measurement of P4. The mean maximum diameter of the dominant follicle (DF) was smaller (P=0.01) in Gyr heifers (10.0 ± 0.8 mm) than in Gyr x HBW (13.0 ± 0.6 mm) or HBW (12.5 ± 0.8 mm). Furthermore, treatment with PGF2α on D0 increased (P=0.02) the maximum diameter of DF (12.9 ± 0.5 vs. 10.9 ± 0.7 mm) in the three genetic groups evaluated. The growth rate of the DF was lower (P=0.008) in Gyr heifers (0.8 ± 0.1 mm/day) than in crossbred heifers (1.3 ± 0.1 mm/day) or HBW (1.2 ± 0.1 mm/day). Heifers of the three genetic groups treated with PGF2α on D0 had higher (P=0.002) growth rates of DF (1.3 ± 0.1 vs. 0.9 ± 0.1 mm/day). The ovulation rate was lower (P=0.02) in Gyr heifers (36.4%) than in crossbred heifers (83.3%), but did not differ from HBW heifers (70.0%). However, higher (P<0.01) ovulation rates (93.7% vs. 35.3%) were observed in heifers treated with PGF2α on D0. The circulating concentration of P4 on removal of the intravaginal device (D8) was higher (P<0.05) in Gyr heifers (3.8 ± 2.2 ng/ml) than in crossbred heifers (2.2 ± 1.2 ng/ml), but no significant contrast was observed in HBW heifers (2.4 ± 1.6 ng/ml). Throughout the treatment period, the circulating concentrations of P4 were lower (P< 0.05) in heifers treated with PGF2α compared to untreated animals. Gyr heifers exhibited a lower growth rate and smaller maximum diameter of DF and a tendency towards a lower ovulation rate compared to HBW heifers. Treatment with PGF2α on the day of insertion of the intravaginal P4 device reduces circulating P4 concentrations and increases the growth rate and maximum diameter of DF, as well as ovulation rate.