Equine Metabolism and Thyroid Hormone

Horses use a variety of body processes to maintain internal body temperature as ambient temperatures plunge. Those include morphological, physiological, biochemical, metabolic, and behavioral adaptations.

For example, many animals stay warm in the winter by decreasing their metabolic rate significantly, leading to a state of hypometabolism, which is more widely referred to as hibernation or torpor.

“Horses do not hibernate, but evidence suggests that certain breeds such as Przewalski’s Horses and Shetlands enter a state of hypometabolism in cold climates as a strategy to stay warm,” shared Kathleen Crandell, Ph.D., a longtime equine nutritionist for Kentucky Equine Research(KER).

In lieu of hypometabolism, other animals increase their metabolic rate to generate heat and stay warm. Unfortunately, lack of available forage in the winter often impairs a horse’s ability to increase its metabolic rate and generate heat.

“Another factor that impacts metabolic rate is thyroid hormone levels, including both T3 and T4,” Crandell shared.

To gain more insight into thyroid hormone levels and metabolic rates during cold weather in the face of limited forage, one research group* recruited Shetland ponies and simulated winter-feeding restrictions. Specifically, during the summer, ponies were fed 100% of their recommended protein and energy requirements and, as winter approached, their feed was gradually decreased to 60% of those levels. Heart rate, activity level, thyroid hormones, and field metabolic rate (FMR) were measured in the summer months and again at the peak of the winter period. Researchers found:

• FMR was higher in the summer than the winter, and significantly lower in the restricted feeding group fed 60% of the recommended energy and protein compared to the control group fed 100% of energy and protein in the winter;
• Similarly, resting heart rate was higher in the summer than winter, and significantly lower in the restricted feeding group in the winter;
• Activity was markedly lower in both groups in the winter, and no difference between the two groups existed; and
• Changes in total T3 concentrations were positively related to changes in FMR, while total T4 concentrations were negatively related to FMR, but neither served as an indicator of FMR.

“Although this study shows that ponies have intact mechanisms for dealing with the cold in times of restricted resources, managed horses don’t need to rely on these mechanisms. Instead, adequate forage generates ample energy and body heat from fermentation in the large intestine,” reminded Crandell.

Many horses can maintain body weight on all-forage diets when fed in sufficient quantities throughout winter. Provide horses with a balancer pellet or a well-fortified vitamin and mineral supplement, like Micro-Max, if no other concentrate is fed. This will ensure all nutrient requirements are met. Micro-Max is available in the U.S. and certain other markets. Australian horse owners should look for Gold Pellet.

Shelter plays an important role in keeping horses warm in winter. Take time during the summer to ensure your horse’s paddocks are ready for winter.

*Brinkmann, L., M. Gerken, C. Hambly, et al. Thyroid hormones correlate with field metabolic rate in ponies, Equus ferus caballus. Journal of Experimental Biology. In press.