Expression of oestrus and timing of ovulation in pigs.

Oestrus is the period around ovulation in which sows show a standing response for boars, thus allowing the boars to mate with the sows. The duration of oestrus may vary between sows from 24 h up to 96 h, which is not related to systemic oestrogen concentrations. The moment of ovulation after onset of oestrus may vary from 10 h to 85 h. Consequently, onset of oestrus is not a good predictor for the time of ovulation. A reliable prediction of ovulation time would be worthwhile, since fertilization results are highly dependent on the moment of insemination relative to the moment of ovulation. However, at present, the duration of oestrus is the best 'predictor' of ovulation time and is independent of the duration of oestrus; ovulation takes place on average 70% of the way through oestrus. The following factors have been found to influence the duration of oestrus (and consequently the optimal time for insemination): boar-stimuli, stress, interval from weaning to oestrus, season and parity. Further study is needed to determine the causes of the variability in oestrus expression between sows, including the regulation of oestrus behaviour at the level of the brain.