Efeito do estresse por calor no desempenho reprodutivo de matrizes suínas nos trópicos e avaliação do limiar de ITU

Abstract

First, the study aimed to model and estimate the impact of the maximum temperature–humidity index (THImax) on the number of stillbirths in sows and to calculate the stress–comfort ratio (S/C). A total of 36,880 farrowings from sows in a commercial farm in Brazil were analyzed. Females were grouped according to parity as gilts, primiparous sows, sows with 2–5 parities, and sows with ≥6 parities. THImax on the day of farrowing was classified as ≤74 (thermal comfort), 75–78 (mild stress), 79–82 (moderate stress), and >82 (severe stress). The effect of THImax on the daily number of stillbirths was estimated using quasi-Poisson regression. The mean number of stillbirths per farrowing and per THImax category was analyzed by ANOVA, and the S/C ratio was subsequently calculated according to THImax categories. Out of a total of 1,826 days, 14.95% were classified as thermal comfort days. Multiparous sows (≥2 parities) showed a higher number of daily stillbirths under THImax >82, whereas primiparous sows showed higher values under THI <74. According to the S/C ratio (severe/comfort), increases in stillbirths of 20% in gilts, 7% in primiparous sows, 14% in sows with 2–5 parities, and 19% in sows with ≥6 parities were observed. The quasi-Poisson model indicated an increase in daily stillbirths in multiparous sows (≥2 parities) under THImax >82, and an S/C ratio greater than 1 across all parity groups, indicating an increased mean number of stillbirths per farrowing, particularly under severe heat stress conditions. In the second study, the objective was to model and estimate the effects of air temperature (Ta) and relative humidity (RH) on respiratory rate (RR), rectal temperature (RT), and surface temperature (ST) of sows, and to assess whether the current THI threshold underestimates or overestimates heat stress under tropical production conditions. A total of 98 sows from a commercial farm in Brazil were evaluated and grouped according to parity as gilts, primiparous sows, sows with 2–6 parities, and sows with 7–11 parities. The effects of environmental variables on thermophysiological responses were estimated using quasi-Poisson regression (RR) and Gaussian models (RT and ST). Increases in Ta and RH significantly affected RR, RT, and ST in sows with 2–6 parities, whereas in gilts, only ST was affected. When estimating RR, this variable was identified as the first physiological response to increase with rising Ta and RH in primiparous and multiparous sows (≥2 parities). In these groups, RR exceeded the physiological threshold (32 breaths·min⁻¹) while THI values were still below 74. In conclusion, heat stress affects sows differently according to parity, and the current THI threshold (74) underestimates the impact of heat stress on sows raised under tropical conditions.