Effect of immunization against progesterone on oestrus, cycle length, ovulation rate, luteal regression and LH secretion in the ewe.

The effects of active immunization against progesterone on reproductive activity were studied in Merino ewes. Immunization against progesterone caused a shortening (P < 0'01) of the interval between ovulations from 17-18 days (controls) to between 6 and 10 days (immunized group); this was associated with a corresponding reduction in the interval between LH surges. The immunized ewes also had higher (P < O· 05) ovulation rates (1' 72) than controls (I. 25) and exhibited a reduced (P < 0'01) incidence of oestrus (26% v. 95%). Many immunized ewes continued to ovulate despite the persistence of corpora lutea from earlier ovulations which led to an accumulation on the ovaries of many corpora lutea of different ages. The frequency of LH pulses in ewes immunized against progesterone (1'8 ± 0·2 pulses/4 h) was significantly (P < 0'001) higher than that of control ewes (0'3 ± 0·1 pulses/4 h). This study highlights the importance of progesterone in the control of oestrus, ovulation, ovulation rate, luteal regression and the secretion of LH in the ewe.