Attainment of puberty in domestic gilts reared under long-day or short-day artificial light regimens

Abstract

The effect of photoperiod on attainment of puberty was examined in four studies using controlled long- or short-day light regimens. Gilts were reared together under natural photoperiod until divided between two controlled environment rooms at a mean age of 106.6 ± 0.52 days on 25 September 1987 (Study 1, n = 32) or 23 March 1988 (Study 2, n = 26). In Study 3 (21 September 1988; n = 35) and Study 4 (21 March 1989; n = 36) the gilts were transferred to the controlled environment rooms at 56.5 ± 0.82 days of age. In all studies the light: dark ratio was initially 12 h light: 12 h dark (12 L: 12 D) in both rooms and it was then either increased or decreased by 10–15 min per week. In Studies 1 and 2 the ratio was increased to 16 L: 8 D or decreased to 8 L: 16 D over 16 weeks. In Studies 3 and 4 it was increased to 14.5 L: 9.5 D or decreased to 9.5 L: 14.5 D over 12 weeks then returned to 12 L: 12 D over a further 12 weeks. Temperature was maintained at 23°C. In Studies 1 and 2 all gilts commenced daily exposure to mature boars at 173.5 ± 0.52 days of age and 82.9 ± 1.63 kg. In Studies 3 and 4 all gilts remained isolated from boars. In Studies 1 and 2 the light regimens during rearing had no significant effect on either the proportion of gilts which reached puberty in response to boars or the interval from first exposure to boars until puberty (overall, long-day 21 29 , 21.9 ± 2.83 days ; short-day 18 29 , 22.1 ± 2.66 days ). However, when gilts remained isolated from boars more of those housed under the short-day regimens reached puberty in both Study 3 ( 10 18 vs. 1 17 , P < 0.01 ) and Study 4 ( 10 19 vs. 0 17 , P < 0.01 ). The mean age at puberty was 209.3 ± 3.13 days in Study 3 and 214 ± 2.61 in Study 4. These data, obtained from gilts housed at constant cool temperatures under controlled light regimens which closely mimicked natural photoperiods, show conclusively that long days are inhibitory to attainment of puberty in the gilt. This finding provides compelling evidence that photoperiod is the major environmental factor responsible for the delay in puberty often seen during the summer under field conditions.