Blanket rule: Light cotton rugs are no match for good shade on a hot day, study finds

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The style of light cotton rug used in the pilot study. Photo: Barbara Padalino
The style of light cotton rug used in the pilot study. Photo: Barbara Padalino

A light-colored cotton rug on a horse is no substitute for decent shade on a hot day, the findings of a pilot study suggest.

Researchers have described an experiment in which they investigated the thermoregulation and stress indicators in horses with and without rugs in typical Australian summer weather.

City University of Hong Kong researcher Barbara Padalino and her colleagues, writing in the Journal of Veterinary Behavior, said horses were potentially subject to thermal stress when temperatures topped 25 degrees Celsius. It is therefore recommended that horses be given shade on hot days.

“However, this is not possible for horses grazing on many Australian rural properties,” the study team noted.

Some horse owners believed, based mainly on anecdotal evidence, that a light cotton rug will reflect sunlight and therefore reduce heat gain in horses.

The study involved 18 healthy university-owned horses, comprising six geldings and 12 mares. The horses were tied in an outdoor arena in direct sunlight for two hours on two different days.

Baseline information was collected at the beginning of the experiment for each horse – their heart-rate, respiration rate, rectal temperature and sweat production, which was scored on a five-point scale. The horses were also monitored for tail swishing, licking-chewing, pawing, and repeated head movements.

Half the horses were then fitted with a light-colored cotton rug and all horses were observed and monitored at regular 15-minute intervals for a further two hours.

The use of the light rug was found to increase rectal temperature and sweat production.

However, unrugged horses were tail-swishing and pawing more, suggesting that the rug had reduced irritation from flying insects.

“Even though wearing a rug did not have an effect on the other parameters, it is worth noting that heart rate, respiration rate and the occurrence of stress-related behaviors were higher than normal values for equids, suggesting that horses were potentially prone to discomfort.

“Overall,” they concluded, “it appears that the use of light-colored cotton rugs may help reduce the irritation caused to horses by flying insects as evidenced by less tail swishing, but may also lead to an increase in internal temperature and subsequently sweat production, increasing the risk of thermal stress and loss of electrolytes.”

They continued: “Wearing a rug is not an adequate substitute for the provision of shade when ambient temperatures exceed 25 degrees Celsius.”

The thermoneutral zone for horses was assessed in a 1998 study as being between 5 and 25 degrees Celsius. Within this range, horses can comfortably maintain their body temperature with little metabolic energy expenditure or negative impact on welfare.

Evaporative heat loss and sweat production significantly increase in horses experiencing thermal stress.

Discussing their findings, the researchers said the significantly higher sweat score in rugged horses suggested that they were less effective in using evaporative heat loss to cool down.

“The wearing of a rug could have limited this natural process by providing a physical barrier to wind factor evaporation.

“The greater rectal temperatures and sweat production seen in rugged horses than in unrugged horses suggests that the light cotton rug may have inhibited the horses’ ability to thermoregulate through the natural wind-mediated evaporation process.

“However, since the values of rectal temperature were still within the normal range for horses, it is possible that both groups of horses were able to adequately thermoregulate regardless, although it could be argued that the rugged horses were under more heat stress.”

Rugging, they said, should not be considered an effective alternative to shade on hot days to prevent heat stress.

“Although there is evidence that horses acclimatise to hot weather conditions, there is still significant risk of heat stress for horses that are unable to dissipate heat effectively, which therefore can compromise the animals’ welfare.”

The study team noted that, in a recent Australian survey on horse management practices, all 505 participants reported providing some form of shelter.

Whilst most provided either natural or man-made shelter within the horse’s paddock, 8% had considered their use of rugs as a form of shelter.

“Bearing in mind that the majority of Australian horses are kept in the paddock on a full-time basis, it can be argued that the provision of shelter should become mandatory and not just recommended, in order to safeguard horse welfare.”

The study team noted that no white horses were used in the study. “Given that we are examining heat reflection, it raises the question of whether the cotton rug would have the same effect on a light-colored animal and whether different types of rug would give the same findings.”

Therefore, the data analysed applied only to dark horses.

The findings suggest that cotton rugs may benefit horses in terms of reducing the impact of flies and other insects on horse comfort and therefore welfare, but this had to be viewed in the context of increasing the rectal temperature and sweating.

“Overall, to safeguard horse welfare, the use of rugs should always be considered in relation to the environmental temperature, the presence of skin disease, the type of management and monitoring the behavioral and physiological parameters of the rugged horse.”

The study was funded by Charles Sturt University, which owned the Thoroughbreds and Standardbreds used in the research.

The study team comprised Padalino, who is with the College of Veterinary Medicine at the City University of Hong Kong; Jaymie Loy and Hayley Randle, with the School of Animal and Veterinary Sciences at Charles Sturt University, Wagga Wagga, New South Wales; and Lesley Hawson, who is with Harness Racing Victoria.

Effects of a light colored cotton rug use on horse thermoregulation and behavior indicators of stress
B. Padalino, J. Loy, L. Hawson, H. Randle
https://doi.org/10.1016/j.jveb.2018.03.003

The abstract of the study can read here.

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