Physiological and behavioral responses of horses to high wet bulb globe temperature and shade access

Agricultural animal care guidelines state that shade should be available when temperatures reach 30°C. The recommendation does not account for variables like humidity, solar radiation, or duration of exposure, which influence thermal load. The objective was to determine whether access to shade or wet bulb globe temperature (WBGT) influence a horse's physiological responses and behavior. Twelve Quarter Horses (13 ± 6 yr, 571 ± 59 kg, 6.0 ± 0.8 BCS) were used to investigate the hypothesis that WBGT or access to shade would affect salivary cortisol (SC), rectal temperature (RT), heart rate (HR), and daily distance traveled. Horses were divided into 3 groups of 4 horses and rotated daily among three 1.5-acre paddocks for 12 d, resulting in 4 repeated rotations. Paddock 1 had 961 ft² of man-made shade available, with food and water only accessible in sun. Paddock 2 had no shade available. In paddock 3, man-made shade was available (961 ft²), with food and water located in shade. Horses were in paddocks 24 h/d and 7 d/wk. The WBGT was recorded in sun and shade. Daily sampling consisted of saliva for SC, collected at 1600 h; RT at 0800, 1200, and 1600 h; continuous HR from 2 horses per paddock from 0800 to 1600 h; and continuous GPS tracking from 0800 to 1600 h. The SC was measured using an enzyme immunoassay (Salimetrics, Carlsbad, CA). Daily distance traveled was approximated as the cumulative distances recorded for all GPS intervals from 08:00–16:00. The GPS data were used to create location heat maps with GIS software. Data were analyzed with a repeated measures mixed ANOVA including fixed effects of paddock, WBGT, and their interaction. When applicable, the repeated measure was time of day nested within day nested within 1 of 4 repeated rotations. Horse nested within group was the random effect, with group as the experimental unit. Differences were significant at P < 0.05. Data are presented as a ranges and medians. The WBGT between 0800 and 1600 h was higher in sun (18–34, 28°C) than shade (16–31, 26°C). Paddock had no detectable effect on SC, RT, or HR. The WBGT had no effect on SC or HR but resulted in a 0.03°C rise in RT for each 1°C increase in WBGT. Horses in paddocks with food and water outside of shade traveled 0.3 km more daily than those with no shade, whereas horses with food and water under shade showed intermediate travel distances. GPS heat maps suggest the increased movement in paddock 1 is driven by horses seeking both shade and access to food and water. The results of this study indicate that horses demonstrate some motivational behavior to use shade in high WBGT conditions. Guidelines should allow for a more nuanced approach to shade provision.