The effect of Temperature-Humidity Index (THI) on the number of stillbirths and stress-to-comfort ratio of sows at different parities in a tropical environment.

It is well known that heat stress negatively affects the productivity, reproduction, and welfare of pigs. However, the role of thermal environments and stress-to-comfort (S/C) ratios in predicting stillbirths has not been studied in tropical regions. Therefore, this study aimed to model and estimate the impact of the maximum Temperature-Humidity Index (THImax) on the number of stillborn piglets and to calculate the S/C ratio. A total of 36,880 farrowings from sows on a commercial farm in Brazil were analyzed. The females were grouped by parity into gilts, primiparous, and multiparous with 2-5 and ≥ 6 farrowings. On the day of farrowing, THImax was classified as follows: ≤74 (thermal comfort), 75-78 (mild stress), 79-82 (moderate stress), and > 82 (severe stress). The effect of THImax on the number of stillbirths per day was estimated using quasi-Poisson regression. The models were adjusted using the quasi-Akaike information criterion (QAIC) and Pearson's residual variance to select the most appropriate model for the data. Also to analyze the mean number of stillbirths per farrowing by THImax category using one-way ANOVA. Then, the S/C ratio was calculated by THImax category. Of the 1,826 total days, 14.95% were classified as comfortable. According to the model, multiparous sows (≥ 2 farrowings) had more stillbirths per day under THImax > 82, while in primiparous sows, ITUmax < 74 is associated with the highest number of stillbirths per day, with a reduction as ITUmax approaches 74. With the severe S/C ratio, an increase in stillbirths per litter was observed in gilts (20%), primiparous (7%), 2-5 parities (14%), and ≥ 6 parities (19%). The quasi-Poisson model indicated an increase in daily stillbirths in multiparous sows (≥ 2 farrowings) under THImax > 82. The severe S/C ratio was greater than 1 in all parity groups, indicating an increase in the average number of stillbirths per farrowing, mainly under THImax > 82.