Methods for detecting heat stress in hutch-housed dairy calves in a continental climate
Heat stress indicators can be classified as animal-based or environmental with optimal indicators depending on the climate of interest . It is unknown whether previously established heat stress thresholds and THI equations translate well for calves in a continental climate or in individual housing situations. The objective of the present study was to establish associations between, and thresholds for, environmental and animal-based indicators of heat stress in Holstein dairy calves individually hutch-raised across a continental climate summer. A secondary objective was to determine the optimal THI equation for use in dairy calves in a continental climate. These objectives were accomplished through assessing correlations between dairy calf animal-based (i.e., RR, RT, and ST) and environmental indicators (i.e., dry bulb and black globe temperature, air speed, RH, and multiple THI equations), equating goodness of fit, and establishing environmental breakpoints at which physiological variables. In this study we hypothesized that animal-based indicators, particularly Skin Temperature, would be strongly correlated to dry bulb Temperature and Temperature Humidity Index, and that environmental breakpoints would be higher than those previously established for subtropical, group-housed Holstein dairy calves due to the greater temporal variation in ambient environment.
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Female Holstein dairy calves (n = 63) were individually housed in sand-bedded polyethylene calf hutches with rear-hutch ventilation. Environmental measurements (dry bulb Temperature, Temperature Humidity Index) were recorded every 15 min. Animal physiological measures including Respiration Rate, Rectal Temperature, and Skin Temperature were recorded thrice weekly at 0700 and 1300 h from 63 individual calves (n = 964 observations). Calves were restricted using wire paneling within their individual hutch for 15 min before assessing Respiration Rate (flank movements for 30 seconds × 2). Restriction was conducted to standardize calf environment, represent hutch usage, and minimize the impact of solar radiation inflating Skin Temperature measures. Correlation, goodness-of-fit, and segmented regression statistical models were employed to estimate optimal heat stress indicators and environmental thresholds for significant changes in dairy calf physiological responses.