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.

Attainment of puberty, cycle lengths, ovulation rate and endocrinology during the periovulatory period were studied in Meishan (MS) and European Large-White hybrid (LW) gilts. The mean age at onset of puberty of 115 days in MS (n = 20) gilts was younger (P < 0.001) than the 235 days in LW (n = 23). In the MS population studied, ovulation rate was not different (P > 0.1) during the third and fourth oestrous cycles, nor were there differences (P > 0.1) in the mean cycle length over the first three cycles. Overall changes in plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH) and oestradiol did not differ significantly (P > 0.1) between the breeds (MS, n = 6; LW, n = 5) during the periovulatory period, but plasma inhibin concentrations were significantly (P < 0.05) higher in the MS. The time intervals from the oestradiol peak concentration and the onset of the LH surge until the onset of behavioural oestrus were significantly different (P < 0.005) between the breeds, with oestrus occurring earlier in the MS. However, no difference (P > 0.1) was found between the groups when the intervals from the peak oestradiol concentration to the onset of the LH surge were compared. These results indicate differences between the breeds, particularly in terms of the age of attainment of puberty and the timing of the onset of behavioural oestrus relative to the oestradiol and LH surges.

Comparisons were made between characteristics of pre-ovulatory follicles recovered from prolific Chinese Meishan gilts (n = 12) and from European Large-White hybrid gilts (n = 13) in the late follicular phase preceding their fifth oestrous cycle, to determine whether there is an ovarian basis for the enhanced prolificacy in the Meishan. A total of 177 follicles per breed was classified as pre-ovulatory, based on follicular fluid oestradiol concentrations. Results obtained demonstrated high variability in all follicular characteristics in both breeds and no decrease in heterogeneity was evident in the Meishan. The Meishan follicles tended to be smaller (P < 0.06) and had less follicular fluid (P < 0.005), but total oestradiol content per follicle was similar (P > 0.1) with the result that the concentration of oestradiol in follicular fluid tended to be higher (P < 0.06) in Meishan than Large-White hybrid pigs. There were no differences between breeds in terms of testosterone concentration in follicular fluid, hCG binding to granulosa cells or total DNA content of granulosa cells. Concentrations of inhibin in follicular fluid were similar in both breeds (P > 0.1) which resulted in a trend towards less total inhibin content in Meishan than Large-White hybrid follicles (P = 0.065). Corpora lutea were recovered from both breeds (n = 12 per breed) on days 27–31 of pregnancy after mating at first, second and third oestrus:corpora lutea were smaller (P < 0.001) and contained less progesterone per corpus luteum in the Meishan (P < 0.05) than in Large White hybrid pigs. However, since ovulation rate was higher in these particular Meishan pigs (P < 0.005), total ovarian progesterone content per animal was similar in both breeds. These results demonstrate that there was no decrease in the variability in follicular characteristics from Meishan pigs. However, both follicles and corpora lutea were smaller in Meishan than in Large-White hybrid pigs, and progesterone per corpora lutea was also lower in the Meishan. In spite of their smaller size, Meishan follicular oestradiol content was similar to that of Large-White hybrid pigs, such that the oestradiol concentration in follicular fluid tended to be higher in Meishan follicles.