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Two experiments were conducted to determine whether oral administration of melatonin alters the onset of puberty in gilts during naturally increasing or decreasing daylength. In Exp. 1, 20 crossbred prepubertal gilts weighing 77.5 +/- .5 kg at 171.8 +/- 1.0 d of age were assigned randomly to receive either a daily oral dose of 3 mg of melatonin (MEL) or ethanol vehicle (ETH) at 1530 from August 31 to December 1, 1987 (decreasing daylength). Gilts were exposed to mature boars for 20 min thrice weekly and blood samples were collected twice weekly. Serum concentrations of progesterone were used to establish age at puberty and length of estrous cycle. In Exp. 2, 20 crossbred prepubertal gilts weighing 67.7 +/- .7 kg at 143.8 +/- 1.1 d of age received either MEL or ETH treatment from February 1 to May 15, 1988 (increasing daylength). Age of puberty was less in gilts that received MEL than in gilts that received ETH in both Exp. 1 (198 +/- 3 vs 228 +/- 7 d; P less than .01) and Exp. 2 (183.8 +/- 2.7 d vs 194.3 +/- 3.3 d; P less than .05). Gilts that received MEL reached puberty at a lighter weight than gilts that received ETH in Exp. 1 (95.6 +/- 2.1 vs 112.4 +/- 3.9 kg; P less than .01) and Exp. 2 (88.1 +/- 1.5 vs 96.0 +/- 1.8 kg; P less than .01). Serum concentrations of LH and FSH, length of estrous cycles, and percentage of muscle of carcasses were similar between MEL and ETH gilts.(ABSTRACT TRUNCATED AT 250 WORDS)

Seventy-six New Zealand white does born between June 28, 1979 and April 6, 1980 were used in an experiment of 2 X 2 X 2 factorial design to determine if 1) exposure to 18 h (L) or 6 h (D) of light/d beginning at weaning influenced age and body weight at puberty and 2) if mating and(or) hormone treatment at puberty influenced ovulation rate and number of embryos. Beginning at weaning, estrus was checked daily using males known to be fertile. At first expression of estrus, does were either mated or nonmated and were injected iv with either 30 IU human chorionic gonadotropin (hCG), or saline. Means for the L and D groups for age at puberty (d), body weight at puberty (kg), ovarian weight (g) and number of embryos 5 d postcoitus were 105 +/- 3.6 vs 121.9 +/- 4.1 (P less than .01); 2.65 +/- .07 vs 2.87 +/- .08 (P less than .05); .56 +/- .04 vs .78 +/- .04 (P less than .001) and 6.1 +/- .83 vs 3.44 +/- .67 (P less than .01), respectively. Photoperiod did not influence number of ovulations. Number of embryos relative to number of ovulations were higher in rabbits exposed to 18 h of light. Does that were both mated and hormone-treated had more (P less than .025) corpora lutea (11.0 +/- .9) than does that were either hormone-treated (7.7 +/- .86) or mated (7.0 +/- .97) only. Mated does had heavier uteri (10.8 +/- .5 vs 7.9 +/- .5) while hormone treatment increased both ovarian and uterine weights (g) over nontreated does (.74 +/- .04 vs .59 +/- .05; 9.9 +/- .45 vs 8.8 +/- .65). Multiple regression analysis revealed that does born in summer reached puberty later than those born during other seasons. It was concluded that season of birth and exposure of does to 18 h of light from weaning affect onset of puberty. Increasing daily light exposure from 6 to 18 h/d may improve litter size in pubertal does.