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Effects of feeding melatonin during proestrus and early gestation in gilts and P1 sows to minimize the effects of seasonal infertility
Seasonal infertility is associated with heat stress and changing photoperiod in summer and fall and is thought to be the cause of delayed puberty, increased wean to estrus interval (WEI), pregnancy failure, and reduced litter size. Research suggests that changing photoperiod determines the seasonality of reproduction by modulation of the function of hypothalamic-pituitary axis and ovary. The aim of this study was to determine if extending the duration of the nighttime increase in melatonin during summer would reduce the incidence of seasonal infertility. Gilts and parity 1 (P1) sows were given oral melatonin once daily beginning in proestrus and extending into early gestation, coinciding with the periods of the follicular phase, corpus luteum (CL) formation, pregnancy recognition and embryo survival. The experiment (Expt.) was conducted at a 6,000 sow breed to wean farm in Illinois. Expt. 1a and 1b were performed in 11 sequential replicates from Jun to Sep. In Expt. 1a, only gilts (n=392) that had expressed a heat-no-serve (HNS) were assigned for study by weight to receive either Melatonin (MEL, 3 mg once daily) or Control (CON) in a syrup solution at 1400 h for 3 weeks starting 1 week before insemination. In Expt. 1b, P1 sows (n=416) were randomly assigned by lactation length and backfat to receive MEL or CON for 21 days starting two days prior to weaning. Data were analyzed for the main effects of treatment and week and their interaction. At this time, preliminary data are only available for estrus expression and returns following insemination. In Expt. 1a there was no effect (P=0.87) of treatment on age at HNS (203d), cycle length (22.3d) or return to estrus (8.9%). However, for cycle length there was an effect of week (P< 0.0001). In Expt. 1b, there was no interaction of treatment with week but there was an effect of treatment (P=0.02) with MEL having a lower WEI than CON (71 vs. 81%) and for return to estrus following insemination was an effect of week (P=0.02) however there was no effect of treatment. At this time, these analyses suggest that supplemental MEL had no effect on the preliminary responses in gilts although farrowing rate and litter size data are needed. In weaned sows, MEL was associated with a 10% reduction in WEI. The mechanism is not clear but follicle measures may help explain this observation with the final farrowing and litter size data.
Keywords: melatonin; seasonal infertility; swine