Context Large-scale genetic evaluation is promising for efficiently improving sow lifetime productivity, and therefore, a reasonable operational model should be pursued to analyse phenotypic data collected from around Japan, where unique seasonality exists. However, the information on establishing the model is currently insufficient. Aims To obtain the fundamental information on using ambient temperature information in developing the model for routine swine genetic evaluation in Japan, by analysing total number born, number born alive, and number stillborn collected at a Japanese farm, together with off-farm ambient temperature measured at the nearest Automated Meteorological Data Acquisition System station. Methods Five repeatability animal models were exploited, considering the effects of farrowing season (Model 1), farrowing month (Model 2), quadratic regressions of daily maximum ambient temperature of farrowing day (Model 3), season and temperature (Model 4), or month and temperature (Model 5). Key results Patterns of the effects of daily maximum temperature of farrowing day estimated using Model 3 were similar to those of farrowing season by Model 1 and those of farrowing month by Model 2. Adding the effect of daily maximum temperature of farrowing day (Models 4 and 5) could explain phenotypic variability better than only considering either of farrowing season or month (Models 1 and 2). Estimated heritability was stable among the models and the rank correlation of predicted breeding values among models was >0.98 for all traits. Conclusions The results indicated the possibility that using public ambient temperature can capture a large part of the phenotypic variability in litter-size traits at birth caused by seasonality in Japan. Implications This study could support the availability of public meteorological data in the development of flexible operational models for future swine genetic evaluation in Japan.

To obtain the fundamental information on using ambient temperature information in developing the model for routine swine genetic evaluation in Japan, we analyzed total number born (TNB), number born alive (NBA), and number stillborn (NSB) collected at a Japanese farm, together with off-farm ambient temperature measured at a nearest Automated Meteorological Data Acquisition System station. Five repeatability animal models were exploited, considering the effects of farrowing season (model 1), farrowing month (model 2), quadratic regressions of daily maximum ambient temperature of farrowing day (model 3), season and temperature (model 4), or month and temperature (model 5). Patterns of the effects of daily maximum temperature of farrowing day estimated using model 3 was similar to those of farrowing season by model 1 and those of farrowing month by model 2. Adding the effect of daily maximum temperature of farrowing day (models 4 and 5) could explain phenotypic variability greater than only considering either of farrowing season and month (models 1 and 2). Estimated heritability was stable among the models and the rank correlation of predicted breeding values between models was >0.98 for all traits. The results indicate the possibility that using public ambient temperature can capture a large part of the phenotypic variability in litter size traits at birth caused by the seasonality in Japan and do not harm, at least, the performance of genetic evaluation. This study could support the availability of public meteorological data in flexible developing operational models for future swine genetic evaluation in Japan.