The Thermoneutral Zone - Another Guest Post By Jason

A number of Melissa’s blog post readers have asked that I do an occasional question and answer series regarding equine nutrition/equine health. I recently had an excellent question posted to me by an astute and caring blog post reader regarding the drop in body condition score that her horse realized over winter this year, so I thought it appropriate that my first question and answer session dealt with the issue of thermoneutral zones in horses.

Equines, like all other mammals including humans, have a thermoneutral zone. This is defined as the range of temperatures in which no extra energy beyond maintenance is required to either heat or cool an animal. The bottom end of this range is called the Lower Critical Temperature (LCT), and the top end of this range is appropriately called the Upper Critical Temperature (UCT). In this post, we’ll mostly deal with the effects of exceeding the LCT.

As a very GENERAL guideline, in humid temperate areas of the United States, horses reach their LCT at around 20 degrees and reach their UCT at somewhere between 60 and 70 degrees. For our purposes here let’s say that the energy required for maintenance on a 1000 lb retiree might be in the range of 18 Mcal (18000 calories) per day.

Would that I could simply post this range, end my post, and fix all the problems in the world, but unfortunately it is quite a bit more complicated than that. SOME of the external factors that affect a horses’ thermoneutral zone can include the following:

1. Area of the country to which the horse is adapted – Horses born and raised in Florida have very different base line thermoneutral zones than do horses raised in Alaska. Generally speaking, when seasonal temperatures fall more than 10 degrees outside the normal range to which the horse is adapted, a horses’ lower or upper critical temperature will be challenged. In middle Tennessee, our typical January high temperature is around 50 degrees, and our typical overnight low is near 30 degrees, giving an average temperature over a 24 hour day of about 40 degrees. As long as ground conditions are firm and it is not raining, most of our horses can handle this sort of weather without any additional grain (or blanketing) beyond what we would normally feed.

In central Ontario, a typical January day might bring a high of 25 degrees and an overnight low of around 5 degrees. Again, if adapted to the climate, and if the weather is dry horses may do well without a blanket in these conditions, but they almost certainly will need additional energy in the form of either a) more hay or b) more grain or c) both ! During moderate, dry cold snaps here in middle Tennessee we like to increase the amount of energy fed by about 10 % (1.8 Mcal/horse/day) using more hay, a little more grain, and supplemental fat when necessary. During extreme cold events, especially with additional adverse environmental challenges, we might supplement energy by 25 % or more.

2. Environmental Conditions – Many things can combine to effectively lower the temperature your horse is feeling. High humidity on a cold day, rain, snow, sunshine, ground conditions ( ie.mud), soil temperature and wind speed all play important roles in determining the comfort of your outdoor horse as well as when it might be necessary to increase the energy in his feed. Just as we feel it more when it is 40 degrees, windy and raining as opposed to 40 degrees in bright sunshine, so do our equine companions. In this case, we always supplement energy in 5 % increments depending on just how nasty it is (or is predicted to get) outside. If appropriately fed, conditioned and provided with adequate wind and rain relief, our equine friends are very hardy and highly adaptable creatures.

3. The condition of your horse !! It’s no surprise that most wild animals go into the late fall and winter slightly over-conditioned if possible. Fat is a great insulator for our wild friends and an excellent energy source (burned to produce heat) too. Thus it should be no surprise that I like to see our retirees a little heavier in the late fall too. Obviously, BCS (body condition score) varies with every horse on the farm, but unless the horse is obese, I like to put one extra condition score on them starting in late September. This is an additional form of insurance for us, and it gives us a wider window in which to correct feeding errors (yes, we make them too !!) BEFORE anything goes seriously amiss.

Hope this is informative ! I kind of enjoy doing these posts !! :)