Effect of Season and Age on Thermophysiological and Hematological Variables of Crossbred Dairy Calves in Tropical Environment

Background: The wellbeing and performance of calves may be impaired if raised in high temperature environments. Physiological and hematological variables serve as a tool to assess the welfare of cattle; therefore, the influence of seasons and age on physiological and hematological variables was evaluated of crossbred dairy calves in a tropical environment.Materials, Methods & Results: Forty-two male and female calves were evaluated from July 2016 to July 2017, when the calves were of 2, 15, 30, and 60 days of age. Respiratory (RR) and heart rates (HR) and rectal (RT) and body surface (BST) temperatures were quantified in the morning. Additionally, in the morning, dry bulb (Tdb) and wet bulb (Twb) temperatures were measured, and then, relative humidity (RH) and a temperature-humidity index (THI) were calculated. Blood was collected to perform hemogram. The day before the calves were evaluated, thermal environment data were collected to verify whether there were cyclic periods of heat stress. The mean air temperature ranged from 19°C to 22.1°C, and the highest THI value of 68 occurred in the summer mornings. In the afternoon, the maximum air temperature ranged from 27.5°C to 29.7°C. In autumn, respiratory and heart rates and body surface temperature were higher in 2-day-old calves than in other ages. The body surface temperature of 60-day-old calves was higher in spring and summer than in autumn. Rectal temperature remained in the normal range throughout the study period. Season did not influence the erythrogram and plaquetogram. The values for red blood cells, MCV, MCHC, RDW, platelets, and MPV varied among the calf ages. Season did not influence the values of leukocytes, monocytes, lymphocytes, or N/L ratio; however, band neutrophils and eosinophils varied among seasons. Band neutrophils and monocytes were not altered by age, whereas leukocyte, segmented neutrophils, eosinophils, lymphocytes, and N/L ratio values varied with age among the calves.Discussion: The air temperature and THI remained within the thermoneutral zone of crossbred dairy calves in the morning; however, in the afternoon the air temperature and THI increased, which indicates cyclic periods of critical heat stress. Higher RR and HR values observed in 2-day-old calves may be due to the physiological changes that accompany adapting to extrauterine life. The RT remained within the reference range for species during all seasons and at all ages, and therefore, the calves were able to maintain normothermia. The air temperature remained lower than the BST and was within the thermoneutral zone; thus, the loss of sensible heat was predominant in relation to evaporation dissipation. Age, breed, time of day, and meteorological variables may influence hematological constituents. Red blood cells of newborn calves are large, of fetal origin, and are replaced by smaller cells with advancing age, which results in a smaller MCV value. The higher neutrophil concentrations in 2-day-old calves resulted in a higher N/L ratio following the trend of plasma cortisol concentration, which is high at birth and decreases with age. The highest values of band neutrophils are because these cells are responsible for phagocytosis of microorganisms and other foreign materials. Since new-born calves are in contact with microorganisms in the environment and are highly susceptible to infections, it is justifiable to observe an increased number of band neutrophils. The differences observed in lymphocyte numbers in calves aged 30 and 60 days during autumn, winter, and spring is likely due to the production of B lymphocytes as an exposure response to agents present in the environment. We conclude that seasons interfere with BST and neutrophil and eosinophil counts, while age affects thermophysiological variables, erythrogram, plaquetogram, and leukogram.