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CIRCULATING OESTROGENS AND PROGESTERONE IN THE BOVINE AFTER SYNCHRONIZATION WITH MELENGESTROL ACETATE

Summary.: Five Friesian maiden heifers and four Friesian or Ayrshire cows were assigned to each of four treatments, namely: (1) controls ; (2) melengestrol acetate (MGA) 1 mg orally daily for 14 days; (3) MGA+2500 i.u. HCG at 72 hr after the last administration of MGA; and (4) MGA+0·5 mg oestradiol benzoate on Day 1 of MGA treatment, +2500 i.u. PMSG at 36 hr+2500 i.u. HCG at 72 hr after the last administration of MGA.

Jugular vein blood was sampled daily over the oestrus period in the control animals and daily after the last dose of MGA until the day after the first oestrus. The concentrations of progesterone and total oestrogens in peripheral plasma samples were assayed by radioimmunoassay without chromatographic separation.

In the control animals, the total oestrogen concentration increased from 7 pg/ml 4 days before oestrus to 14 pg/ml on the day of oestrus, then decreased to 9 pg/ml 1 day after oestrus. Treatment with MGA alone did not alter this pattern at the first synchronized oestrus. Administration of HCG 72 hr after MGA withdrawal apparently depressed total oestrogen concentration. This was counteracted by prior treatment with PMSG. A comparison of the data from animals receiving MGA only and from controls indicates no adverse effect on release of oestrogens from ovarian follicles after cessation of MGA treatment.

The progesterone concentration in control animals decreased from 9 ng/ml 4 days before oestrus to 0·35 ng/ml on the day of oestrus. The MGA-treated animals had low progesterone values for several days before oestrus. Three of the sixteen animals treated with gonadotrophins had progesterone concentrations higher than basal values.