Your horse’s body may be doing a lot more to prepare for cold weather than just growing a winter coat.
I recently read a very interesting study on Przewalski horses, a primitive breed, that shows there are seasonal influences which are independent of calorie intake or environmental temperature: https://jeb.biologists.org/content/209/22/4566.long . Many of our easy keeper/EMS prone breeds may retain these mechanisms for coping with cold which would lead to overfeeding them if we automatically increase calorie intake for cold.
This always leads to questions about whether we should let our horses go through a natural weight gain and loss cycle by essentially starving them seasonally. I personally don’t think this is any healthier than when humans see-saw their diet and weight. In the natural environment their bodies are so stressed by winter that their organs actually shrink – and some die. The “natural” cycle is geared toward one thing – survival – and secondarily to breeding and survival of the fetus. We don’t have to go to that extreme.
One interesting finding though is that metabolic adaptation to winter isn’t totally a reflection of poor quality diet in winter (see the last paragraph of that article). Metabolism begins to slow down in advance of winter and feral animals eat less even if provided with more food. We may need to adjust caloric intake lower in winter rather than higher. However, this German study found that reduced metabolic rate in ponies in winter was largely associated with underfeeding https://www.ncbi.nlm.nih.gov/pubmed/28689503
It’s also interesting that in many animals insulin resistance is part of their winter survival strategy. It even occurs in people https://www.ncbi.nlm.nih.gov/pubmed/23235713 . In horses, insulin response to glucose is affected by season https://www.ncbi.nlm.nih.gov/pubmed/28701287 . There were no convincing seasonal changes in intravenous combined insulin glucose test responses in normal horses in southeastern USA https://www.ncbi.nlm.nih.gov/pubmed/22594619 but have to wonder what effect the warm climate might have been having since there are many observational reports of erratic insulin readings in cold weather. This study found insulin highest in October in grazing horses in NE USA https://www.ncbi.nlm.nih.gov/pubmed/30911753 (colder months not studied).
I’m not sure if any conclusions can be drawn from all of this but points to consider are:
A. The presence of IR in an animal may reflect a more primitive energy metabolism, geared to survival (the “thrifty gene” theory)
B. Increased IR may be part of the survival mechanism in cold weather, along with a programmed reduction in basal metabolic rate that precedes the onset of cold weather (??? timing aligns with the seasonal ACTH rise ???)
C. For the very IR horse/primitive breeds we may need to restrict sugar/starch more stringently in fall and winter, possibly even calories as well if the animal is not being exercised to force a higher metabolic rate. If the horse is shivering, put on a blanket rather than increasing food!
Eleanor Kellon, VMD