The effect of dose and route of oestradiol benzoate administration on plasma concentrations of oestradiol and FSH in long-term ovariectomised heifers

Oestradiol (E2) suppresses FSH and affects follicle wave dynamics in cattle. However, neither the optimum dose of ODB required to suppress FSH nor the effect of route of ODB administration on blood concentrations of E2 are known; hence, the aim of this experiment was to answer these questions. Ovariectomised heifers received Progesterone Releasing Intravaginal Device (PRID) for 7 days, and 4 days later heifers received one of eight ODB treatments at second PRID insertion as follows; (1) 0.0 mg (Control; n=3), (2) 0.5 mg (n=4), (3) 1.0 mg (n=4), (4) 2.5 mg (n=6), (5) 5.0 mg (n=4), (6) 10.0 mg (n=4), (7) 5.0 mg (n=4), and (8) 10.0 mg (n=5). For treatments 2–6 inclusive, ODB was administered intramuscularly in oil, while for treatments 7 and 8, the ODB in powder form was administered topically in the vagina by gelatine capsule attached to the PRID. Blood samples were collected every 6 h for the first 48 h, every 12 h for the next 48 h, and twice daily for a further 6 days. The interval from ODB administration to peak E2 concentration was similar (P>0.05) for treatments 2–6 where ODB was administered intramuscularly (mean 13.4±1.24 h), and was longer (P<0.05) for the intravaginal capsule treatments (mean 25.5±2.84 h). Plasma concentrations of E2 increased with increasing intramuscular dose of ODB injected, (plasma E2=−0.237+16.109 (dose)−0.74 (dose)2, R2=0.75; P<0.05). Peak plasma concentrations of E2 following the 5- and 10-mg capsules were similar to each other and to those following the 0.5-mg injection (P>0.05), but were lower than concentrations obtained following injection of 1.0–5.0 mg (P<0.05). Across all treatments, both the maximum percentage decline in FSH and the interval to FSH nadir were related to the peak plasma concentrations of E2 (maximum % decline in FSH=11.17+1.564 (peak E2)−0.009 (peak E2)2, R2=0.75; P<0.01), (hours to FSH nadir=10.628+1.486(hours to peak E2)−0.0282(hours to peak E2)2, R2=0.22; P<0.05). Concentrations of FSH increased as E2 declined from its peak value, irrespective of maximum value achieved. It was concluded that the intramuscular administration of ODB in oil to ovariectomised heifers given a PRID results in higher plasma concentrations of E2 and causes a greater reduction in FSH than administration topically by intravaginal gelatine capsule. E2 transiently suppresses FSH in ovariectomised heifers, and the magnitude of the suppression is dose-dependent; however FSH concentrations begin to increase 1–2 days after ODB administration while concentrations of E2 were declining but still high.