Cervical mucus and oestrous behaviour responses to oestrogens in sheep: Progesterone-oestrogen relationships and role of progesterone pre-treatment

The final dose of progesterone (5, 10, 20 mg) and time to oestrogen injection relative to the final dose of progesterone (24–72 h) had no significant effect on the production of cervical mucus measured 24 h after the injection of 30 μg oestradiol benzoate (ODB). However, there were significant effects on the behavioural oestrous responses (time from injection of oestrogen to onset of oestrus and duration of oestrus). Time to onset of oestrus increased from 18 to 27.8 h with increasing dose of progesterone (P < 0.001) and decreased from 24.8 to 20 h with increasing time to oestrogen injection (P < 0.05). Conversely, the duration of oestrus decreased from 36.2 to 23.8 h with increasing dose of progesterone (P < 0.001) and increased from 29 to 39 h with increasing time to oestrogen injection (P < 0.01).Ovariectomized ewes became refractory to ODB as measured by the cervical mucus response after the fifth sequential daily injection of 20 μg oestradiol benzoate. Progesterone priming was not required to restore subsequent sensitivity to oestrogen treatment. However, there was a positive linear relationship between length of recovery period and level of response to subsequent treatment.It was concluded that: (1) progesterone pre-treatment or priming is not necessary in the cervical mucus bioassay in ovariectomized ewes; and (2) a period of 8–16 days is needed between assays for normal sensitivity to be regained.