Melatonin implants do not alter estrogen feedback or advance puberty in gilts

• We determined effects of season and melatonin on estradiol action in castrated gilts.
• We determined the effect of season and melatonin on the time of the first ovulation.
• LH and FSH increased with age but was unaffected by season or melatonin.
• Puberty occurred earlier in gilts born in spring compared to winter.
• Treatment of intact gilts with melatonin failed to advance the age of puberty.

Puberty in pigs is often delayed during late summer and autumn, with long daylength the most likely cause. We hypothesised (1) that gilts born around the shortest day would have a later release from the negative feedback actions of estradiol than gilts born around the spring equinox and (2) melatonin treatment would result in an earlier release from estradiol negative feedback and advance the onset of puberty in gilts born around the spring equinox. We first determined the optimal number of estradiol implants required to monitor the release from estradiol negative feedback in ovariectomised gilts. Secondly we determined whether melatonin implants altered negative feedback in 4 cohorts of ovariectomised gilts born between the winter solstice and spring equinox, and in the following year whether melatonin altered the time of the first ovulation in 5 cohorts of intact gilts born between the winter solstice and spring equinox. Plasma LH and FSH increased between 126 and 210 d of age (P < 0.001) in each cohort (season), but there was no effect of cohort, melatonin treatment or interactions (P > 0.05). Age at first detection of elevated plasma progesterone in untreated, intact gilts decreased across the 4 cohorts (P < 0.05). Melatonin treatment of intact gilts failed to advance the age of puberty irrespective of their season of birth (P > 0.05). In conclusion, while we confirmed that estradiol sensitivity is decreased as gilts age, we failed to demonstrate any effects of season or melatonin on estradiol feedback or melatonin on puberty.