The Chill of the Iron-Clad Foot
The Brainy Hoof Blog by Christa Lesté-Lasserre
Don’t you just love snuggling up to a warm and cosy iron or steel plate?
Yeah, well, horses probably don’t either.
In fact, Norwegian researchers recently stumbled across an interesting trend about hoof temperatures in shod and unshod horses. While checking the surface temperature on different parts of the body on 21 horses using thermography, they discovered that the horse wearing iron shoes consistently had colder feet than the horses without shoes.
“I have always thought that shod hooves felt colder when handling them, and I’ve seen barefoot hooves leave a melting mark in the snow. So even though I expected it a bit, I was really surprised to get significant differences between shod and unshod hooves in radiant surface temperatures.” Grete H.M. Jørgensen, PhD, of the Norwegian Institute of Bioeconomy Research, in Tjøtta.
You don’t need a fancy scientific thermometer to check the differences in radiant surface temperatures between shod and unshod hooves, although the measurements scientifically confirm the difference. The temperature difference is significant enough to feel it with a bare hand, Jørgensen says.
Since warmth in the hoof could indicate health issues - and in particular, laminitis - the natural heat of barefoot hooves surprise some people, including health professionals. “Barefoot feet feel much warmer to the touch, up to a point where veterinarians experienced with shod horses could be worried and suspect illness in the barefoot individuals,” Jørgensen explains.
Pictures from an infrared camera comparing the temperature of the rump and hoof of a shod warmblood horse (left) and an unshod coldblood horse (right)(Meisfjord Jørgensen, Mejdell and Bøe, 2020)
That’s not to say that owners of barefoot horses shouldn’t ignore excess heat in their horses’ hooves, of course. It’s just that one possibility of that warmth is simply that it’s normal for a shoeless hoof. Scientists haven’t studied what’s considered ‘normal’ barefoot temperature and ‘unhealthy’ barefoot temperatures, although that’s the ‘next level’ of research following the Norwegian team’s discovery according to Jørgensen.
The Physics of Cold Feet
In simple terms, anything that is cold tries to ‘grab’ heat from whatever happens to be nearby. Some metals, such as copper and aluminium, conduct heat, meaning they transfer heat from one surface to another. That is why these metals are great for stove-top cooking!
Iron and steel, though, are poor conductors of heat and actually ‘absorb’ the heat around them and keep it for themselves before conducting it, little by little, elsewhere.
To get an idea of this for yourself, you can place a steel or iron horseshoe on the ground under the summer sun next to a ‘horseshoe’ you make yourself using some crumpled up aluminium foil. Then take them into the shape (use gloves to protect your hands if necessary) and see how long it takes for them to cool off. You’ll realise the horseshoe takes a long time for it to ‘give up’ its heat, steel and iron like to grab heat and hold onto it.
But regardless of the metal - steel, iron, aluminium or even copper (although, you wouldn’t have a copper horseshoe) - the rules of thermodynamics suggest that heat in the hoof would be conducted away from that hoof and into the metal, whether to stay a while in the metal or to move on to the ground below it. It’s just that some metals are better at conducting heat than others.
Either way, the bottom line is the same, the hoof gives its heat away through the shoe and thus becomes cooler, as Jørgensen’s study indicates.
You can test this for yourself, too. In the winter, you can try putting one hand flat on the ground and the other hand flat over a steel or iron horseshoe (at room temperature to start) on the ground, and wait a while. You might find that the hand on the shoe feels colder faster. It’s not because the steel is colder than the ground, but the steel is channelling your heat away from your hand.
A Disruption of Thermoregulation?
Horses have a unique, complex thermoregulation system that keeps their vital bodily organs at the right temperature despite having long limbs that don’t help keep the heat concentrated in the central body. To meet that challenge, horses have a sort of ‘radiator system’ in the limbs, Jørgensen says.
“Arteries and veins run close together in the legs so that the warm arterial blood from the body core can warm up the cooler, deoxygenated blood (that is coming back towards the heart from the feet) in the returning veins,” Jørgensen explains.
The Equine Circulation System
(Horse & Hound, 2017)
That heat transfer from the arteries to the veins actually makes the legs and feet cooler than the body core, she says. However, the advantage of that is that the arterial blood then arrives in the body core warmer than it otherwise would have. “It reduces the temperature of the distal limbs and also reduces total heat loss, to save and preserve heat to the more important body parts and organs,” she says.
What effects that might have on foot health or general health - if any at all - is yet to be determined, of course. After all, the heat loss doesn’t seem to be very dramatic, the scientists say. But what is clear so far, is that metal shoes do seem to be associated with colder feet.
“An iron shoe will probably involve increased heat loss from the hooves,” says Jørgensen. “This might be negative in really cold conditions, or in horses living longer periods in the lower parts of the thermoneutral zone (temperatures that don’t require extra energy use to regulate body temperature, but close to or sometimes crossing over lower critical temperature).”
Graph showing the difference between shod and unshod hooves in IR surface temperature.
(Meisfjord Jørgensen, Mejdell and Bøe, 2020)
You can read their full study here: https://www.sciencedirect.com/science/article/pii/S0306456519302578?via%3Dihub
Passionate about horses and science from the time she was riding her first Shetland pony in Texas, Christa Lesté-Lasserre writes about scientific research that contributes to a better understanding of all equids. After undergrad studies in science, journalism, and literature, she received a Master's degree in creative writing. Now based in France, she aims to present the most fascinating aspect of equine science: the story it creates.
Horse & Hound, 2017. Combating Filled Legs. [image] Available at: <https://www.pressreader.com/uk/horse-hound/20170921/281814284042839> [Accessed 23 September 2020].
Meisfjord Jørgensen, G., Mejdell, C. and Bøe, K., 2020. Effects of Hair Coat Characteristics on Radiant Surface Temperature in Horses. Journal of Thermal Biology, [online] 87. Available at: <https://www.sciencedirect.com/science/article/pii/S0306456519302578?via%3Dihub> [Accessed 23 September 2020].