Summary. Blood samples were collected from primiparous sows via indwelling jugular cannulae at 15-min intervals for 12 h before and for 24 h (2 sows) or 48 h (10 sows) after weaning and then every 4 h until behavioural oestrus. Weaning to oestrus intervals ranged from 3 to 10 days and 2 sows showed no signs of oestrus and had not ovulated by Days 11 and 16 after weaning.
Prolactin concentrations in plasma decreased significantly (P < 0·001) and reached basal levels 1–2 h after weaning in all sows whilst plasma progesterone concentrations remained basal until ~30 h after the preovulatory LH surge in sows that ovulated. Elevated concentrations of prolactin or progesterone during the post-weaning period were, therefore, not responsible for delayed restoration of cyclicity.
Overall, mean LH concentrations rose significantly (P < 0·001) from 0·22 ± 0·02 during the 12-h period before weaning to 0·38 ± 0·03ng/ml during the 12-h post-weaning period. After weaning, pulsatile and basal LH secretions were markedly increased for sows that showed an early return to oestrus (≤ 4 days) compared with sows showing a longer weaning to oestrus interval but a correlation did not exist between either of these LH characteristics and the time taken to resume cyclicity. Mean LH concentrations before weaning were, however, inversely related (r = − 0·649; P < 0·05) to the weaning to oestrus interval.
Overall, mean FSH concentrations rose significantly (P < 0·001) from 151·1 ± 6·2 (s.e.m.) ng/ml in the 12-h period immediately before weaning to 187·7 ± 9·7ng/ml in the subsequent 12-h period but there was no correlation between FSH concentrations, before or after weaning, and the interval from weaning to oestrus. However, a significant correlation was apparent between ovulation rate and peak concentrations of the rise in FSH after weaning (r = 0·746; P < 0·05) and overall mean FSH values (r = 0·645; P < 0·05).
It is concluded that both LH and FSH concentrations in peripheral blood rose in response to removal of the suckling stimulus at weaning. The increase in LH pulse frequency associated with weaning was not directly related to the weaning to oestrus interval although a specific pattern of LH secretion was observed in sows showing an early return to oestrus ( ≤ 4 days). It is proposed that the characteristics of LH secretion after weaning may reflect the ovarian status at the time of weaning and that this may, in part, be dependent on steroid feedback. The weaning-associated rise in FSH may be involved in the determination of ovulation rate.