Detecting heat stress: Examination of temperature-humidity index thresholds for respiration rate and body temperature in barn- and pasture-housed peripubertal dairy heifers

If animals cannot dissipate accumulated heat from metabolism and the environment, heat stress (HS) occurs. Temperature-humidity index (THI) thresholds for peripubertal dairy heifers are not well defined, despite their known susceptibility to HS. Establishing THI thresholds in heifers housed in heat-abated freestall barns and on pasture will provide practical management metrics and inform future HS research. This study included 2 experiments in a humid continental climate: one under natural seasonal conditions and another in climate-controlled chambers. The objectives were to (1) establish THI thresholds for HS in peripubertal dairy heifers under different housing types, (2) develop prediction equations for respiration rate (RR) and body temperature in heifers using THI as the predictor variable, and (3) determine if a climate-controlled environment could result in similar biological responses and THI breakpoints as observed in peripubertal heifers housed in summer pasture. In the first experiment, RR, vaginal body temperature, and THI were recorded twice daily for a total of 14 d in 12 peripubertal heifers (291 ± 19 kg BW, 281 ± 6 d of age; mean ± SD). Each heifer experienced 7 d of housing in a naturally ventilated barn with fans above the stalls and 7 d of outdoor pasture housing, in a crossover fashion. Simple linear and segmented regression were used to develop prediction models for RR and body temperature, using THI as the predictor variable. Simple linear regression best modeled RR and body temperature response to THI in heifers housed in a barn with fans above the freestalls when THI ranged from 64 to 82. In contrast, a segmented regression with a THI threshold of ∼74 best reflected RR and body temperature response to THI for pasture-housed heifers when THI ranged from 62 to 88. After THI reached the threshold of 74 without remediation, signs of HS escalated abruptly. A second experiment was conducted to determine if housing in a climate-controlled chamber could result in similar biological responses and THI breakpoints as observed in heifers housed in summer pasture. For this, RR and body temperature relative to observed THI were monitored twice daily in a subset of 5 heifers from experiment 1 (340 ± 16 kg BW; mean ± SD) that were housed in a climate-controlled chamber. In this setting, THI was increased incrementally every 24 h over a 5-d period, from ∼65 to 80. Similar to pasture housing, RR and body temperature abruptly increased with each unit increase in THI beginning at THI of 74 for RR and 76 for body temperature. Collectively, these experiments confirm that peripubertal dairy heifers exhibit classic signs of HS, with RR and body temperature increasing sharply near THI 74 in both pasture and climate-controlled environments. Heifers housed in environments with heat abatement features do not respond to increased THI in the same manner; biological responses to increased THI remain linear. Regardless of housing environment, heifers weighing ∼325 kg should be monitored more closely for signs of HS as THI approaches 74 to minimize the risk of HS-related production losses.