Seasonal variation in attainment of puberty in isolated and boar-exposed domestic gilts

Abstract

The effect of season on natural and boar-induced puberty in the domestic gilt was studied using a total of 467 Large White/Landrace gilts in 15 replicates over a 19-month period from October 1987 to May 1989. The gilts were allocated to either isolation from mature boars ( n=13−18, N=234) or exposure to a mature boar (>18 months old) for 30 min day −1, 7 days week −1 ( n=14−17, N=233). Boar exposure or checking for oestrus in the absence of boars commenced at an overall mean age and weight of 164.8±0.18 days and 85.0±0.41 kg. Commencement of these procedures marked the start date for each replicate. Treatments continued until puberty or slaughter at 225 days. The reproductive tracts of all gilts were recovered at slaughter to confirm their reproductive status. A higher proportion of boar-exposed than isolated gilts reached puberty by 225 days of age (0.798 vs. 0.298, P<0.001) and the mean interval from commencement of oestrus detection until puberty was shorter (26.8 vs. 45.1 days, P<0.001) for the boar-exposed gilts. Puberty was severely inhibited in replicates which started in the late spring and summer months. Cluster analysis based on the mean hours of light per day in the period around the commencement of each replicate separated the replicates into either a long-day (>12 h day −1, N=9 replicates) or a short-day (<12 h day −1, N=6 replicates) group. In both the isolated (0.139 vs. 0.526, P<0.001) and the boar-exposed gilts (0.740 vs. 0.894, P<0.01) the proportion reaching puberty was lower for the long-day group (replicates which commenced between October and March) than for the short-day group (replicates which commenced between April and September). There was no seasonal effect on the interval to puberty in either treatment. Body weight at the start of each replicate did not differ between gilts which reached puberty and gilts which did not in either the isolated gilts (prepubertal ( n=156) 84.5±0.51 kg vs. pubertal ( n=78) 85.7±0.63 kg, P>0.10) or the boar-exposed gilts (prepubertal ( n=47) 83.9±0.99 kg vs. pubertal ( n=186) 85.3±0.48 kg, P>0.10) and there was no seasonal trend in initial body weight. The data shows clearly that attainment of puberty in the domestic gilt was strongly affected by season. Natural onset of puberty in isolated gilts was delayed and the efficacy of boar exposure for stimulating puberty was reduced in replicates which received a mean daylength in excess of 12 h day −1 in the period around 165 days of age. These data support the conclusion that photoperiod is the major environmental factor which mediates the seasonal variation in attainment of puberty in the domestic gilt.