Physiological, behavioral, and serological responses of horses to shaded or unshaded pens in a hot, sunny environment1

Abstract

Housing recommendations for horses invariably include providing access to shade on hot, sunny days, but the potential benefits have not been scientifically studied. This experiment measured physiological, behavioral, and serological responses of horses confined individually to completely shaded (SH) or completely unshaded (SUN) drylot pens during the summer in Davis, CA. Twelve healthy adult horses in a crossover design experienced both treatments for 5 d each. Rectal temperature, respiration rate, skin temperature, and sweat scores were recorded hourly from 1230 to 1730 h daily. Observations were recorded from 1200 to 1800 h for proximity to water, foraging, locomotion, and insect avoidance behaviors. Daily blood samples were obtained to measure cortisol, hematocrit, and neutrophil-to-lymphocyte ratio. Automated and handheld sensors were used to record environmental conditions. The mean ambient temperature from 1200 to 1800 h during the study was 30.6 °C. Rectal temperature was greater for horses in SUN than for SH (mean 37.8 °C and 37.5 °C, respectively, SE = 0.06, P = 0.002) as was respiration rate (25.5 and 20.5 breaths/min, SE = 1.3, P = 0.008), and skin temperature (35.6 °C and 34.6 °C, SE = 0.1, P< 0.001). Horses in SUN showed sweat in 51.4% of observations vs. 1.1% for horses in SH. Horses in SUN spent more time than SH horses standing near their water source (34.0% of observations vs. 20.2%, SE = 0.3, P = 0.004). No differences were observed for foraging, locomotion, or insect avoidance behavior (P > 0.05). Cortisol concentrations were greater in SUN than SH (3.4 and 2.6 μg/dL, respectively, P < 0.001) but remained within the normal range for resting horses. No treatment differences were observed for hematocrit or neutrophil-to-lymphocyte ratio (P > 0.05). Horses exhibited treatment differences in the physiological measures first, most notably in rectal temperature at 1230 h, corresponding to peak solar radiation. Behavioral responses followed these physiological changes, with treatment differences in time standing near water becoming apparent at 1400 h as ambient and black globe temperature increased. Our results indicate that both the SH and SUN treatment groups exhibited thermoregulatory responses to these summer conditions and horses benefited from shade, as it mitigated these physiological and behavioral changes. These results are applicable in developing best management practices for the care of domestic horses.

Preference of domestic horses for shade in a hot, sunny environment1

“…shade structure. They were able to maintain consistent RT, RR, and SK and showed almost no sweat, in contrast to our previous study that found elevated levels of each of these variables in horses without access to shade in sunny, hot conditions (Holcomb et al, 2013). However, their use of shade did not coincide temporally with maximum mean T BGT as hypothesized.…”

“…However, until recently, there have been no studies directly evaluating the effects of shade on horses. In a previous study (Holcomb et al, 2013), individually housed horses without access to shade in a hot, sunny environment showed greater rectal and skin temperatures and respiration rates than completely shaded horses. Although this provides evidence that horses benefit from shade, a focused preference test is required to evaluate whether or not they will take advantage of this resource when use is optional.…”

“…Ambient temperature (T amb ), relative humidity (rH), black globe temperature (T BGT ), and solar radiation were recorded at 5-min intervals for 24 h/d during all trials using sensors with automated data loggers as described previously (Holcomb et al, 2013). Surface soil temperature (T soil ) was measured hourly from 1300 to 1700 h using a handheld infrared thermometer (Raytek MT6, Raytek, Santa Cruz, CA) at 2 locations each in the shaded and unshaded areas.…”

“…Rectal temperature (rT), respiration rate (rr), skin temperature (SK), and a sweat score were measured as described previously (Holcomb et al, 2013) at 0900, 1230, and 1800 h daily. Blood samples were obtained on d 0 and 5 (Trials 1-3) and on d 7 (Trials 2 and 3) at approximately 1800 h. Samples were processed as de-scribed previously (Holcomb et al, 2013) to measure hematocrit (HCT), the neutrophil to lymphocyte ratio (n:L), and cortisol.…”

Preference for shelter and additional heat in horses exposed to Nordic winter conditions

“…Horse were categorised into 5 breed categories according to the presence of Warmblood (10) and Coldblood (12) breed characteristics [21], and height at the withers and estimated body weights as follows: ponies (height: ≤119 cm; body weight: 230-313 kg) = 2; small Coldblood horses (120-149 cm, 234-464 kg) = 7; large Coldblood horses (≥150 cm; 432-596 kg) = 3; small Warmblood horses (120-149 cm; 372-402 kg) = 2; large Warmblood horses (≥150 cm; 414-650 kg) = 8. Body size and surface relative to body mass influence heat loss to a great extent [3].…”