The Influence of Photoperiod on the Onset of Puberty in the Female Rabbit

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.