Oestrus is the period around ovulation in which sows show a standing r
esponse 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 ovu
lation 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 fer
tilization 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 independe
nt of the duration of oestrus; ovulation takes place on average 70% of
the way through oestrus. The following factors have been found to inf
luence 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 reg
ulation of oestrus behaviour at the level of the brain.