FACTORS AFFECTING THE AGE AT ONSET OF PUBERTY, OVULATION RATE AND TIME OF OVULATION IN CHINESE MEISHAN GILTS

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 Meish an gilts. Gilts approaching puberty either in the spring (n = 88) or i n the autumn (n = 40) were housed indoors under natural daylight condi tions and observed daily for oestrous behaviour. Gilts approaching pub erty in the spring were younger (P < 0.001) and more likely to reach p uberty by 100 days of age (P < 0.01) than were those approaching puber ty 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 tent h to twenty-first (n = 17) oestrous cycle and in primiparous Meishan s ows (n = 12) by counting the number of corpora lutea or corpora albica ntia at laparoscopy, laparotomy or at ovarian recovery following slaug hter. Ovulation rate increased (P < 0.001) with reproductive age and a pproached that of primiparous sows only when gilts had experienced gre ater than or equal to 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 hC G 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 o estrus, gilts administered hCG ovulated earlier than did the control g roup (44.2 versus 48.7 h, P < 0.05) and at an interval of 42.2 h relat ive to the administration of hCG.