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The timing of oestrus, the concentrations of oestradiol, LH and progesterone, and embryo survival on day 35 of pregnancy in 16 weaned multiparous sows were studied in relation to the moment of ovulation. Ovulation was detected using transrectal ultrasonography. The interval between weaning and ovulation varied between 126 and 214 h (156 ± 29). The peak oestradiol concentration was 27 ± 17 pg ml–1; sows with a shorter interval between weaning and ovulation had higher peaks of oestradiol (r = –0.54, P = 0.02). The interval between the peak oestradiol concentration and peak LH concentration was 11 ± 4 h. The shape, width and height of the LH surge were variable, but did not influence the interval between the peak LH concentration and ovulation, which varied between 26 and 34 h (30 ± 3). The interval between ovulation and the time when the progesterone concentration rose to more than 1.0 ng ml–1 above basal progesterone concentrations varied from 6 to 19 h (13 ± 4). Ovulation took place at a more or less constant relative time of the oestrous period (after 72 ± 8% of the duration of oestrus), irrespective of the duration of oestrus. The timing of the peak concentrations of oestradiol and LH and the rise in progesterone concentrations were closely related to the occurrence of ovulation but not to the onset of oestrus. Embryo survival on day 35 of pregnancy was related to the interval between the peak concentrations of oestradiol and LH (r = –0.68, P = 0.02); shorter intervals resulted in higher embryo survival. A similar relationship was found between embryo survival and the interval between the peak oestradiol concentration and the rise in progesterone concentration (r = –0.59; P = 0.05), and between embryo survival and the interval between the peak LH concentration and ovulation (r = 0.60; P = 0.03). In conclusion, the interval between the peak LH concentration and ovulation was on average 30 ± 3 h and was relatively constant between sows (from 26 to 34 h). The concentrations and temporal changes in oestradiol, LH and progesterone profiles differed among sows. The temporal relationships between hormonal changes and ovulation seem to be of importance for embryo survival and require further investigation.
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