As the temperature drops, horses burn more energy to stay warm, therefore their energy requirements will increase. We can provide our horses extra energy in the form of concentrates and forage. Forages are digested by the microbes in your horses large intestine and produce more heat than concentrates. A great analogy is to compare concentrate and forage to your wood stove. Concentrates are your kindling and Forage are the logs. You need the kindling to start the fire, but when you go to bed at night and you want that fire to maintain heat throughout the night...you toss on those large logs.
How much extra forage does your horse need to stay warm?
Ideally a horse would receive free choice hay throughout the winter. Then they can regulate if they need more or less during the day and night. Some of us have easy keepers who would not do well on free choice hay! For these horses, we need to regulate their intake. Weigh out their hay and provide it in small hole hay nets to prolong their amount of chew time.
The first thing you need to do is find out how much your horse weighs. I recommend using the following method versus a weight tape that only wraps around the heart girth. Using body length plus heart girth tends to be more accurate.
Using a soft measuring tape (the kind often found in sewing kits) measure your horses heart girth and write the number of inches down.
Next, measure the body length from point of shoulder to point of hip, and write it down.
Here is the math formula: Heart Girth (HG) x Heart Girth x Body Length (BL) / 330 = weight
Example: Max has a Heart Girth measurement of 75" and a Body Length measurement of 68".
75 x 75 = 5262 (HG x HG)
5262 x 68 = 382500 (total x BL)
382500 / 330 = 1,159lb
Make sure you write it all down to refer to throughout the winter. Write down the HG and BL also, this will help ensure you that you are measuring at the same location each time. If you are a few inches off, most likely you are measuring from a slightly different location (unless you can tell by looking that your horse has obviously gained or lost weight). Try to find landmarks. Paint horses have great landmarks!
Now that you know how much your horse weighs, you can figure out how much hay he needs to eat. Your average horse in ideal weight should consume 2% of his body weight. Some hard keepers require upwards of 2.5% of body weight. If you have an overweight horse, you can drop down to 1.5% of his body weight, but no less than that or you will be creating a whole other set of issues!
Max weighs 1,159lb and needs to eat 2% of his body weight. 1159 x .02 = 23lb of hay per day.
Getting back to the original question... When the temperature drops below 45 degrees F (including wind chill) horses start to burn extra energy to stay warm. This 45 degree mark is called "Critical Temperature". For every 1 degree F below the critical temperature, your horse will require a 1% increase in digestible energy (DE). Think of digestible energy like your horses calorie requirement. As with everything horse related, there are a lot of variables to this rule, such as wind chill, rain/sleet, your horses coat thickness or if you blanket.
Since horse nutrition is 90% math, lets get back to that! Here is your math formula: Critical temperature - actual temperature = percent increase in digestible energy required.
If I continue with the math we would calculate your horse's DE requirements, next calculate how much extra DE is needed for your current temperature, next test your hay to see exactly how much DE it provides per pound, lastly calculate how much extra hay will provide the additional DE required for the current temperature. However, since I've taken up this much of your time, and hopefully you will go out, weigh your horse and calculate how much hay he needs (1.5%, 2% or more), I'll skip to the long awaited answer:
Your horse will need roughly 1/2-1lb extra hay per day for every 5 degrees the temperature drops below 45 degrees F.
If it's 20 degrees F where our buddy Max lives he would need an additional 2 1/2-5lb of hay.
This guideline is for horses at maintenance or light work. Additional hay will be needed if your horse is a hard keeper, in heavy work, or on poor quality hay.
Stay warm and Happy Feeding!