No abstract

The timing of ovulations in 42 PMSG-treated ewes was determined by repeated endoscopy. The first ovulation occurred at a median time of 23 . 6 +/- 0 . 5 (s.e.m.) h after the onset of oestrus. The median interval between first and second ovulations was less than 1 h, and that between first and last ovulations was approximately 6 h. In 59 untreated ewes, probit regression analysis was applied to the number of ovulations which were found by endoscopy to have occurred by 23, 25 and 27 h after the onset of oestrus. The median time of first ovulation was 25 . 5 +/- 0 . 5 h after the onset of oestrus, this interval being similar in single- and twin-ovulating ewes. The median interval between twin ovulations was 1 . 2 +/- 0 . 6 h. Ovulation occurred after the end of oestrus in at least 75% of ewes.

Oestrus was consistently induced by 20, 40 or 80 microgram oestradiol benzoate in progesterone-primed lambs aged 23 weeks and 17 weeks respectively in two experiments. The duration of oestrus increased linearly with increasing log dose of oestrogen, and was negatively correlated with the time of its onset. In the absence of progesterone there was a reduced incidence, later onset and shorter duration of oestrus. Progesterone alone did not induce oestrus. In lambs treated during January (Exp. 2), a later date of injection of oestrogen was associated with earlier onset and longer duration of oestrus. The induced oestrus was anovulatory. Oestrogen reduced the proportion of lambs with follicles greater than or equal to 3 mm in diameter whilst progesterone had no effect. Lambs which were the progeny of low- and high-fecundity dams did not differ in their oestrous or ovarian responses. Correlations between the dam's lamb production index and the time to onset and duration of the induced oestrus were also not significant.

Plasma LH and testosterone levels did not differ significantly between high and low fertility rams before or after sexual stimulation (ejaculation or teasing). Repeated stimulation caused significant elevation of mean plasma LH and an almost significant rise in testosterone concentration. Plasma testosterone peaks followed those of LH after 30-60 min. A single sexual stimulation did not always cause an LH peak or increase the mean LH level.

The difference between fresh undiluted ram semen from high (H) and low (L) prolificacy Romney selection lines in success in fertilizing artificially inseminated Romney, half-Romney and Cheviot ewes (unrelated and randomly allocated) was examined in 3 experiments. For ewes producing natural multiple ovulations, the fertilization rate was 71% (40/56) and 40% (20/50) for semen from H rams and L rams respectively (P < 0.01). Fertilization rates for PMSG-induced multiple-ovulating ewes were 61% (11/18) for H semen and 22% (4/18) for L semen (P > 0.05). Values for single-ovulating ewes were 52% (35/67) for H semen and 50% (28/56) for L semen (P > 0.05).