Influence of climatic conditions in early life: a systematic review of behavioral, physiological, performance and health responses of pre-weaning calves

Calves are more thermotolerant to heat stress due to lower metabolic heat production compared to dairy cows, however, they are still susceptible to its effects. This review aimed to systematically examine the effects of heat stress on behavior, physiology, performance, and health of pre-weaning dairy calves. A comprehensive literature search was conducted in the Web of Science and Scopus databases. After applying a four-step PRISMA assessment to the 233 initially identified studies, a final selection of 16 studies was analyzed. Microclimate variables and thermal comfort indicators were the most frequently investigated topic, discussed in 94 % of the studies. Physiological responses of thermoregulation was addressed in 69 % of the studies, while 44 % focused on behavior, 25 % on performance, and 13 % on health parameters. Our review characterizes and highlights the various benefits of maintaining dairy calves under thermal comfort conditions. Calves housed under thermal comfort conditions were exposed to an ambient temperature 5ºC lower and a temperature-humidity index 7.33 points lower than those in heat stress conditions. Under thermal comfort, calves showed increased milk (~0.660 g/day) and concentrate intake (~0.314 g/day), as well as improved average daily gain (~0.060 g/day). Additionally, reductions were observed in rectal temperature (~0.22ºC), body temperature (~1.77 ºC), respiratory rate (~12.21 breaths/min), and heart rate (~17.7 beats/min) on calves at thermal comfort conditions. These findings provide strong evidence that housing calves in thermally comfortable conditions benefits their overall performance. However, data on calf health and social behavior under different environmental conditions remain scarce and inconclusive, highlighting the need for further research in this area.