Ovulation as Determined by Transrectal Ultrasonography in Multiparous Sows: Relationships with Oestrous Symptoms and Hormonal Profiles

Summary Twenty‐four cross‐bred multiparous sows in their second and fourth parities were studied during two consecutive oestruses after weaning. Careful oestrous control was done on the animals and ovulation was monitored by transrectal ultrasonography. Blood for analysis of LH and E 2 contents was collected through an indwelling catheter. The sows were mated/inseminated once approximately 18 h before expected ovulation in their second oestrus. The interval between weaning and ovulation varied from 86 h‐204 h (153 ± 26). Onset of standing oestrus to ovulation was found to be longer in sows with long oestruses and ovulation occurred within a mean of 68 % of the oestrous period. Duration of the first pro‐oestrus was significantly shorter than the second pro‐oestrus (40 h and 71 h, respectively). There was no significant difference within sows for the intervals from onset of oestrus to ovulation during the first and second oestrus after weaning (37 h and 39 h, respectively). The diameter of the largest follicles at onset of oestrus and at ovulation had very small variation between animals. The mean intervals from E 2 peak to onset of oestrus and to ovulation were lh (range −10‐+h22h) and 44 h (range 34–54 h) respectively, and from LH peak to onset of oestrus and to ovulation 12 h (range −10‐+32h) and 35 h (range 27–48 h), respectively. The present study indicates that repeated ultrasonographic examination of ovaries during one oestrus can be used to predict the time of ovulation during the subsequent oestrus. On the other hand, the large variations of the intervals from E 2 peak to onset of oestrus and to ovulation, and also from LH peak to onset of oestrus and to ovulation, make it difficult to predict accurately the time of onset of oestrus and ovulation using endocrine parameters.