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Differences in adaptive response to heat stress comparing heat‐tolerant and heat‐sensitive cattle over 24‐hour periods

Little is known about the heat adaptive ability of bovine raised in different thermal environments. Heat‐sensitive Angus steers (ANG) from Missouri (MO;n=5) and Oklahoma (OK;n=5) were tested with the heat‐tolerant Romosinuano breed (RO;n=5) from Florida in environmental chambers at the University of Missouri. Steers were maintained at thermoneutrality (TN; 19–22°C) through Day 8, followed by 2 weeks of cycling heat stress (HS; 26–36 and 30–40 °C for ANG and RO, respectively). RO were kept at a higher ambient temperature (Ta) to incur a similar level of HS as ANG. Respiration rate (RR), and both rectal (Tre) and skin temperatures, were measured every hour for 24 hours at TN, Early Heat (EH), and Late Heat (LH). All animals increased (p<0.05) RR and Tre from TN to HS. MO decreased (p<0.05) Tre and RR from EH to LH by 0.18°C and 9.5 bpm, while OK exhibited no differences (p>0.05). RO had similarities with OK by showing no difference in Tre (p>0.05) from EH to LH. However, RO decreased RR (p<0.05) from EH (102.2 bpm) to LH (84.3 bpm) similar to MO. Likewise, there were temporal skin temperature differences during HS for ANG but not for RO. Results suggest that there are regional differences in bovine response to heat stress that are related to their climatically diverse environments. It is possible to identify phenotypic markers that characterize adaptation levels to heat stress across breeds. USDA Agreement No. 58‐6227‐3‐016