Our objective was to examine interactions between climatic factors and either parity or total number of pigs born for the occurrence of sows having stillborn piglets (SBP) and on the number of SBP, during two seasons. Datasets included 161,299 parity records from 101 herds located in a humid subtropical climate zone; all the sows farrowed either between June and September (hot and humid season) or between December and March (cold season). The climate data were obtained from 21 weather stations located close to the herds. For each parity record, average daily maximum (HT) and minimum temperatures (LT) for the farrowing date and the previous day were coordinated with the performance data. Multilevel logistic regression and Poisson regression models were conducted for whether or not a sow had SBP in a given litter, and for the number of SBP by sows that had farrowed at least one SBP, respectively. The occurrences (±SEM) of sows having SBP during the hot and humid season and during the cold season were 49.6±0.17% and 48.8±0.18%, respectively. For sows that had farrowed at least one SBP, mean SBP was 2.0±0.01 pigs. In the hot and humid season, the occurrences of parity 3 or higher sows having SBP increased by 2.1-2.7% as HT increased from 25 to 30°C (P<0.05), but HT was not associated with the occurrences of parity 1-2 sows having SBP (P=0.73). Also, a sow exposed to 30°C had 0.04 more SBP than one exposed to 25˚C (P<0.05). In the cold season, the occurrences of sows having SBP in any parity group increased by 0.9-2.2% as LT decreased from 5 to 0°C (P<0.05). Also, as LT decreased from 5 to 0°C, the occurrences of sows having SBP and the number of SBP for sows that farrowed 16 pigs (90th percentile) increased by 2.7% and 0.09 pigs, respectively (P<0.05). Taking into account parity and total number of pigs born, HT in summer and LT in winter appear to be critical for the occurrence of SBP and for the number of SBP.
Keywords: Cold stress, hot weather, relative humidity, stillbirth