In Table 3, the data are presented according to parity, previous lactation length and
weaning to estrus interval on the incidence of ovarian cysts. Although only 1% of sows had
lactation length as long as 14 days, 26.7% of these sows had ovarian cysts. This indicates
that cyst incidence is significantly greater when lactation period is shorter than 14 days.
According to Sesti and Britt (1993), lactation can be divided into the following phases
according to its endocrine status: hypergonadotropic phase (from farrowing to the second
to third day), hypogonadotropic or transition phase (from the 3rd to 14th) and normalization
phase (after the 14th day). According to these authors, if pigs are weaned in the first phase,
the follicles present at farrowing can continue to grow because LH and FSH secretion is not
suppressed. However, as sows are unable to produce a preovulatory surge release of LH,
these follicles tend to grow into cysts. If sows are weaning during the hypogonadotropic
phase, LH and FSH will be suppressed. According to Estienne et al. (1997), there is a
GnRH secretion inhibition induced by the production of endogenous opioids released by
the suckling stimulus. The exact cause and pathogeny of ovarian cysts, although not fully
determined, is associated with LH release defficiencies (Close and Liptrap, 1975; Miller,
1984). Stress situations, such as the one found in early weaning, could induce the release
of adrenocorticotropic hormone and according to Miller (1984) daily administration of
adrenocorticotropic hormone in the follicular phase can produce ovarian cysts.