Follicle Size and the Process of Ovulation in Sows as Studied with Ultrasound

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In several experiments, transrectal ultrasound was used to count the number of follicles on both ovaries every half hour for the last 5 to 20 hours before ovulation. In 13 spontaneously ovulating sows, the deviation between the maximum follicle count and the corpora lutea count at 100 hours after ovulation was zero in 54% of the ovaries (14 of 26), 1 in 27%, 2 in 15% and more than 2 in 4% of the 26 ovaries (Soede et al., 1992). This deviation was not related to the corpora lutea count. Therefore, transrectal ultrasound offers a reliable method to count the number of pre-ovulatory follicles and assess the duration of ovulation. Slaughter of sows before, during or after ovulation as monitored with ultrasound, confirmed the accuracy of the technique as a tool to follow the process of ovulation. Results of experiments that study the duration of ovulation can only be used when the procedures used do not interfere with the processes involved. Effects of transrectal ultrasound on the duration of ovulation can not be studied directly, since no other noninvasive method is available. Therefore, in an experiment in which follicle growth and ovulation time in sows were synchronized, Soede and Kemp (1993a) used non-scanned control sows to trace effects of scanning on indirect parameters such as fertilisation rate, embryonic development, embryo diversity and accessory sperm count at Day 5 after ovulation. No effects of repeated transrectal ultrasound were found on any of the parameters studied. It was concluded, that transrectal ultrasound offers a reliable method for assessment of the duration of ovulation in sows and does not seem to interfere with the processes studied. In Table 1, a summary is given of the experiments in which the duration of ovulation was assessed. The duration of ovulation varied between sows from 1 to at most 7 h, but normally did not last longer than 2 to 3 hours. In the experiments, it was found that neither artificial insemination at 22 and 30 h after hCG (Exp. 3), nor stress conditions as caused by 2months of tethering (Exp. 4), nor advanced ovulation by decreasing the interval between PMSG