Effect of thermal amplitude on physiological parameters, ruminal fermentation, digestibility, health, and performance...

This study aimed to evaluate the effects of daily thermal amplitude during the first 28 d of age and its residual impact until 90 d of age on respiration rate (RR), heart rate, rectal temperature (RT), intake, ruminal fermentation, nutrient digestibility, blood metabolites, immune markers, health status, and performance of Holstein dairy calves. Thirty-four dairy calves were individually housed in a climate-controlled chamber and randomly assigned to 1 of 2 treatments: control (exposed to constant temperature and humidity index (THI) of 66), or thermal amplitude (TA) exposed daily to THI of 66 (0330 to 0630 h), 84 (0630 to 1530 h), 66 (1530 to 1830 h), and 54 (1830 to 0330 h) from birth to 28 d age (exposure period [EP]). From 29 to 90 d of age, all calves were maintained in identical housing conditions, spanning the postexposure period (PEP). Intake, physiological responses, and health outcomes were measured daily, performance, and ADG were evaluated weekly, whereas ruminal and blood parameters were assessed biweekly, and blood cytokines were evaluated on d 28. Apparent digestibility was measured at 9 to 12 d (digestibility 1) and 23 to 26 d of age (digestibility 2). During EP, TA calves exhibited increased RR, higher RT with a greater number of days in hyperthermia, and elevated milk and water intakes. Additionally, the TA showed a greater ruminal acetate:propionate ratio, and fecal nitrogen on digestibility 2. Interferon gamma induced protein (IP-10) and interleukin 8 (IL-8) were reduced in TA calves, and interleukin 4 (IL-4) increased. During PEP, residual effects in TA calves included increased RT, water intake, ruminal pH, blood cholesterol and creatinine, and reduced ruminal acetate and total short-chain fatty acids. Despite physiological and metabolic alterations, no differences were observed in health status or growth performance between treatments. These findings suggest that although thermal amplitude challenges thermoregulation in early life, it does not impair calf development under proper management. However, calves exposed to thermal amplitudes may not fully develop thermoregulatory mechanisms during periods of high temperatures, leading to occasional residual effects during PEP.