Delayed age of gilts at first mating associated with photoperiod and number of hot days in humid subtropical areas

The objective of the present study was to quantify the associations between age at first mating (AFM) in gilts and the climatic factors of photoperiod (PP; h), number of hot days (HD) and relative humidity for different herd productivity groups. This study used records of 37,362 gilts born in 2007 and 2008 in 101 Japanese herds, which were classified into high-performing and ordinary herds based on the pigs weaned per mated female per year. The climate data were obtained from 21 weather stations. The HD was defined as the number of days that achieved a maximum temperature >25 °C. Average values of daily PP, relative humidity and HD from day 91 to 150 after birth of a gilt were coordinated with the respective gilt performance data. Two-level mixed-effects models were applied to the data by using a herd at level 2 and a gilt at level 1. Mean AFM (ranges), PP, HD and relative humidity were 247.9 days old (152–364 days old), 12.2 h (9–15 h), 18.7 days (0–60 days) and 68.4% (48–87%), respectively. Delayed AFM was associated with decreased PP, more HD and being in an ordinary herd (P < 0.05), but not with relative humidity. As PP rose by an hour, the AFM in high-performing herds decreased by 1.13 days rather than that in ordinary herds. It is possible that AFM in replacement gilts could be hastened by improving light control and cooling management during hot days.