This study examined the effects of season on genetic parameters for production and reproductive traits and quantified within contemporary group effects of temperature on these traits using linear and plateau-linear regression models. From 2003 onwards, data were available on similar to 60 000 gilts for the routinely recorded production traits (BF: back fat; LADG: lifetime average daily gain) and similar to 45 000 litters for the sow reproductive traits (TB: total born; NBA: number born alive; BWT: average piglet birthweight). A subset of gilts were also recorded for test period daily gain (TADG), daily feed intake (ADI) and feed conversion ratio (FCR) and, later, as sows (n similar to 2000) for average daily lactation feed intake (LADI). Least-squares means for some production and reproductive traits significantly differed between seasons: summer and winter means were 2.28 +/- 0.017 vs 2.54 +/- 0.011 kg/day for ADI, 2.80 +/- 0.022 vs 3.21 +/- 0.011 kg/kg for FCR, and 1.61 +/- 0.02 vs 1.54 +/- 0.02 kg for BWT. However, some statistically significant differences (due to large n) were biologically insignificant. Trait variation also differed between seasons, but heritability estimates did not significantly differ from each other. Heritabilities were (summer vs winter): BF: 0.43 +/- 0.03 vs 0.41 +/- 0.02; LADG: 0.18 +/- 0.02 vs 0.16 +/- 0.02; TADG: 0.12 +/- 0.10 vs 0.08 +/- 0.06; ADI: 0.37 +/- 0.15 vs 0.22 +/- 0.07; FCR: 0.14 +/- 0.11 vs 0.17 +/- 0.06; TB: 0.09 +/- 0.01 vs 0.10 +/- 0.01; NBA: 0.06 +/- 0.01 vs 0.07 +/- 0.01 and BWT: 0.37 +/- 0.03 vs 0.32 +/- 0.04. Genetic correlations between the same trait recorded in different seasons were generally very high (> 0.70), with the exception of TB, where the genetic correlation between spring and autumn was 0.65 +/- 0.09, suggesting a genetic component to the effect of seasonal infertility on litter size. Regression models demonstrated that two selection lines had different responses to increasing temperature, despite concurrent selection in the same environment. Plateau-linear models were generally better than linear models for describing changes to production traits with temperature. Based on maximum temperature at the end of performance testing, the estimated temperature thresholds above which lifetime growth performance was compromised were 25.5 and 32.5 degrees C in the two lines. There were only small linear relationships between reproductive traits and temperature. Overall, the ongoing acclimatisation to the thermal environment and the partial confounding of contemporary group with temperature variables (season explained 62% of variation in average daily temperature) are potentially contributing factors to the lack of major differences in heritability estimates between seasons, and the relatively small regression coefficients for the effects of temperature on performance. Nevertheless, temperature can be demonstrated to affect phenotypic outcomes within contemporary groups using commercial data.
