Effect of terminal sire line on heat stress responses in growing-fattening pigs selected for optimal growth rate vs optimal carcass quality

Management of heat stress in animal husbandry is becoming more important as the number and intensity of heat waves increase due to climate change. Pigs are more sensitive to heat than other animals but show individual differences. Could these differences be traced to sire lines used in Europe? In the present study, the influence of terminal sire line on heat stress-related parameters was investigated to better understand genotype-environment interactions with the aim of improving heat stress resilience in commercial growing-finishing pigs. In total, 360 fattening pigs in three consecutive batches were divided at weaning into pens of five piglets of the same genetic background. The animals were progeny of TN70 sows × a terminal sire line selected for either optimal growth rate (S_growth, homozygous stress negative MaxiMus sire line) or optimal carcass quality (S_carcass, homozygous stress positive Belgian Piétrain sire line). Pigs were monitored from 10 weeks of age (25.2 ± 0.4 kg) until slaughter age (118.4 ± 0.7 kg) during the summer of 2022. Respiration rate, rectal temperature and passive behaviour were evaluated during days with a high heat load (predicted outdoor temperature ≥ 28 °C, Temperature-Humidity index (THI) ≥ 75) and thermoneutral days (THI < 75). Blood parameters were evaluated in batch 3 during an artificially induced 7-day heat wave at 21 weeks of age. The expected phenotypic differences between the terminal sire types could be confirmed, with faster growth in S_growth pigs (1 003 vs 923 g/day, P < 0.001) and better carcass quality in S_carcass pigs (67.3 vs 61.7 mm lean meat thickness, P < 0.001). Rectal temperature and respiration rate showed no significant differences among sire lines as heat load increased (ΔTHI × sire line, P > 0.05). Regardless of genetic background, physiological parameters were influenced by weight: heavier pigs showed faster increases in rectal temperature and respiration rate with rising THI in comparison to lighter pigs (interaction term ΔTHI × weight, P < 0.001). Daily gain of S_carcass pigs dropped as heat load duration increased, whereas daily gain of S_growth pigs either remained fairly stable or increased (sire line × NDT (number of days when THI ≥ 77 within a week), P = 0.020). Future research should focus on identifying a terminal sire line that combines steady growth under high heat loads with optimal performance traits such as feed conversion ratio and carcass quality.