A study was carried out to compare preovulatory ovarian events in prolific Meishan gilts (which have high levels of embryo survival) with contemporary white hybrid control gilts. Gilts of similar reproductive ages (second or third oestrous cycle) were observed three times a day for oestrous behaviour and ovaries recovered at a time estimated as within 7 h of ovulation (white hybrid, n = 13; Meishan, n = 16). Preovulatory follicles were recovered (n = 195, white hybrid; n = 252, Meishan), and oocytes were aspirated and fixed for later identification of meiotic stage, and follicular fluid was frozen for subsequent determination of progesterone. The number of presumed ovulatory follicles recovered per animal was similar to the expected ovulation rate (15.0, white hybrid; 15.75, Meishan; P > 0.1); however, follicles from Meishan gilts were smaller (8.1 versus 9.0 mm, P < 0.001) and contained less follicular fluid (139.9 versus 187.6 μl, P < 0.02) than did white hybrid gilts. Total follicular progesterone (88.7 ng per follicle, white hybrid; 77.4 ng per follicle, Meishan) and concentration of progesterone in follicular fluid (526 ng ml⁻¹, white hybrid; 640 ng ml⁻¹, Meishan) did not differ between the two breeds (P > 0.1). There were no breed differences in the extent of heterogeneity in follicular diameter, volume of follicular fluid, and total follicular progesterone and progesterone concentration (P > 0.1). However, classification of recovered oocytes into seven discrete meiotic stages revealed that more oocytes recovered from Meishan preovulatory follicles were in the more advanced stages of meiosis than were those recovered from white hybrid gilts (P < 0.001). In conclusion, it is suggested that the advanced oocyte maturation in follicles of Meishan pigs before ovulation may be important for ensuring the prolificacy of this breed.

Experiments were carried out to study: the effects of season on age at puberty, the influence of reproductive age on ovulation rate, and the time interval from the onset of oestrus to ovulation in Chinese Meishan gilts. Gilts approaching puberty either in the spring (n = 88) or in the autumn (n = 40) were housed indoors under natural daylight conditions and observed daily for oestrous behaviour. Gilts approaching puberty in the spring were younger (P < 0.001) and more likely to reach puberty by 100 days of age (P < 0.01) than were those approaching puberty in the autumn. Ovulation rate was estimated in gilts at second (n = 22), third (n = 24), fourth (n = 18), fifth to ninth (n = 9) and tenth to twenty-first (n = 17) oestrous cycle and in primiparous Meishan sows (n = 12) by counting the number of corpora lutea or corpora albicantia at laparoscopy, laparotomy or at ovarian recovery following slaughter. Ovulation rate increased (P< 0.001) with reproductive age and approached that of primiparous sows only when gilts had experienced ≥ 10 oestrous cycles (19.2 versus 21.0). The time interval between the onset of oestrus and ovulation was studied in six naturally cyclic Meishan gilts and nine Meishan gilts administered hCG at the onset of oestrus. All gilts were observed six times a day for the commencement of oestrous behaviour and were subsequently examined by laparoscopy at 32 h following onset of oestrus and every 8 h until ovulation, which was at a maximum of 56 h. Relative to the onset of oestrus, gilts administered hCG ovulated earlier than did the control group (44.2 versus 48.7 h, P < 0.05) and at an interval of 42.2 h relative to the administration of hCG.

Two experiments were carried out in which follicular aromatase activity was compared between Meishan and Large-White hybrid gilts. In Expt 1, preovulatory follicles (n = 10 largest per animal) were recovered from Meishan and Large-White hybrid gilts (n = 5 per breed) on the day before predicted onset of behavioural oestrus, and the granulosa cells and theca tissue incubated to determine aromatase activity. Follicles recovered from Meishan pigs were smaller (P < 0.01) and contained fewer granulosa cells (P < 0.05), but follicular oestradiol content of the breeds was similar (P> 0.1). Aromatase activity was higher in the theca tissue (P < 0.05) and tended to be higher in the granulosa cells recovered from Meishan follicles (P = 0.065). In Expt 2, granulosa cell aromatase activity was investigated during the early follicular phase (estimated day 16 of cycle) in Meishan and Large-White hybrid gilts (n = 6 and 5, respectively). The number of follicles ≥ 1 mm diameter recovered per animal was 171 for both breeds (P>0.1), whereas the number of follicles ≥2 mm diameter was 65 and 101 (P < 0.05) from Meishan and Large-White hybrid gilts, respectively. The mean diameter of all follicles recovered was smaller in the Meishan gilts (P< 0.001). Overall, neither the number of granulosa cells per follicle, as indicated by DNA estimation, nor the oestradiol content differed between the breeds at this time (P>0.1). Mean aromatase activity was assessed in granulosa cells recovered from alternate follicles ≥3 mm diameter and was higher in the Meishan gilts when expressed per μg DNA (P ≤ 0.025). As follicle diameter increased, the difference in aromatase activity between the breeds became more pronounced (P < 0.05). These results indicate that Meishan follicles have higher aromatase activity in the granulosa and theca cells during both the early and late follicular phase, compared with Large-White hybrid follicles. Whether this has a role in ensuring the prolificacy of the Meishan pig requires further investigation.