Guide to Horse Rugs: When, What and; How Many

Ask the Vet: Nikki Pursey

Sep 22, 2024

The Great Rugging Debate

When to start rugging? What rugs to use? How many rugs to put on?

Rugging is a contentious issue, and I have witnessed many heated debates on this topic in yards. At this time, some horses might already have three rugs on, while others remain without any. So, what's the correct approach for our four-legged friends?

The Science of Equine Thermoregulation

How Horses Naturally Cope with Cold

The first thing to consider is how different species cope with the changing weather. Whilst we may be utterly miserable, the horse has developed some unique mechanisms for coping with the cold and wet weather.

What is the Thermal Neutral Zone (TNZ)?

The thermal neutral zone (TNZ) is the range of temperatures in the immediate environment at which a healthy adult animal can maintain a normal body temperature without needing to use energy beyond its normal basal metabolic rate. For humans, the TNZ is about 18-30°C, but for horses, it is between 5-25°C; the lower and upper temperatures of this range are referred to as the lower and upper critical temperatures, respectively (LCT and UCT). So, for the vast majority of unclipped adult horses, there is no need to reach for a rug until the temperature drops to about 5°C. Foals, older horses and those who have been clipped are a different story.

The horse’s average core body temperature is between 37.5-38.5°C (99-101°F), and it is important that you know what is normal for your horse. Checking the rectal temperature with a thermometer is the best way, but placing a hand on the horse’s shoulder or trunk is the next best thing; your horse should feel warm to the touch. Feeling a horse’s ears to check if it is warm is a very poor indicator of temperature regulation.

Mechanisms of Heat Loss in Horses

There are four ways in which heat can be lost from the body:

Evaporation - this is the main mechanism of heat loss for both humans and horses. Sweat or water evaporates from the skin, lowering the body’s temperature. Heat loss from respiration also occurs (although not via panting as in other domestic species).
Radiation - the emission of heat waves that may then be absorbed, reflected or transmitted through a colder body. The amount of heat lost from the skin is linked to the ambient air temperature and body mass. Smaller horses have a higher body surface-to-mass ratio, and therefore, lose heat more readily. A variation of radiation is radiant heat loss - this is when radiation (in the form of infrared radiation) is lost to solid surroundings (e.g. a solid-walled stable); this explains why keeping your horse in a stable in cold weather can actually increase the amount of radiant heat loss.
Conduction - where heat is lost by direct physical contact. Water conducts heat very well, so heat is rapidly lost from wet skin.
Convection - where heat is lost by the movement of airflow across the skin, e.g. standing in an exposed field can result in heat loss this way.

Factors Influencing a Horse's Need for Rugs

Internal body temperature is maintained by a huge range of metabolic activities, including digestion, movement and breathing, and heat is also conserved by the insulating properties of hair, muscle, fat and skin. The delicate balance between heat generated and heat lost is what maintains the internal core temperature. Domestication, however, has changed this delicate balance. A full winter coat is not only designed to be insulating but also water-resistant to some degree; pilo-erection of the coat causes warm air to be trapped next to the skin, adding another layer of insulation.

There are several other factors to consider when assessing a horse’s ability to effectively manage its core temperature:

Considering Age and Body Condition

Age - young (and sometimes very old) horses do not have the same level of insulation and are more prone to heat loss, and therefore, have much higher LCTs. Also, studies have shown that aged horses have an increased susceptibility for overheating, especially during exercise. This is due to age-related changes in the physiological mechanisms important for thermoregulation.
Body condition - underweight horses will have decreased muscle and fat mass, resulting in reduced insulation. Meanwhile, overweight horses have extra insulation in the form of fat, and therefore, utilise less of their ingested energy for heat production; this excess energy is subsequently converted into fat, resulting in a self-perpetuating cycle.
Activity level - immobile horses, or those left in restricted paddocks where they cannot walk significant distances, will have reduced metabolic activity, resulting in reduced heat production.

Breed Differences in Thermoregulation

Breed - warmblood types have greater levels of surface heat loss compared with cold-blooded types, but it is not known if this is due to body composition or differences in hair coat thickness

The Impact of Clipping on Heat Loss

Clipping - clipping results in increased heat loss from exposed areas of skin. Furthermore, the neck, shoulder and trunk are the areas from which most surface heat is lost, as these are the areas most commonly clipped.

Making Informed Decisions on Rugging Your Horse

So, how does this help you decide what to do about rugging your horse? There is no easy answer to this, and every horse needs to be treated as an individual, and its need for rugging (or not) assessed by considering all of the factors listed above. It is important not to let our own feelings of being cold result in over-rugging of our horses.

Assessing Individual Needs

Horses in good body condition, who have a full coat, access to forage, free-roaming exercise and with some form of shelter do not necessarily need any rugs, but those who are old, young, exposed to harsh weather conditions or liable to getting wet for long periods may require some form of help. Climate change will also impact when, and if, a horse needs to start wearing a rug.

At the end of the day, my advice is to be sensible; a horse will avoid conditions which promote discomfort or heat loss, so monitor them closely and adapt your management to subtle changes in your horse's behaviour but be mindful of the natural weight gain and weight loss cycle over the seasons.

The Risks of Over-Rugging

It is now widely acknowledged that over-rugging is a far bigger welfare issue than under-rugging as it is a major factor in horses gaining and subsequently failing to lose weight. Obesity is probably the biggest welfare issue facing our equine population today.

About the Author:
NIKKI PURSEY | MA VetMB CertAVP(ESM) MRCVS
Farr & Pursey Equine Veterinary Services

Nikki graduated from Cambridge Vet School in 2007, having already gained a degree in Zoology in 2004. She went straight into working in equine first opinion practice, initially in Devon before moving to the Home Counties in 2009. In 2015, she founded Farr & Pursey Equine Veterinary Services alongside a former colleague, Ricky Farr MRCVS.

Farr & Pursey Equine is now a three-vet ambulatory, first opinion equine practice covering Herts, Beds and Bucks, and offers some of the most advanced diagnostic equipment in the area. Nikki’s primary area of interest is in equine reproduction and she has undertaken further studies in this subject.

As a result, in 2016, she was awarded the Certificate of Advanced Veterinary Practice in Equine Stud Medicine. She also has a keen interest in equine dentistry and preventative healthcare. When not at work, Nikki enjoys competing in obstacle races and mud runs.