It's been -18 up here already over Xmas. Horses are fine they can cope with cold better than you think. It's when they are also wet that they struggle.
Here's a pic of some TBs in the snow when I was in the USA this time last year. They lived out, unrugged, 24/7. At one point it was -25 during the day, no idea how much colder at night. They were happy and fine, in fact I never even saw them use their field shelters.
QR - As SpottedCat says, horses can easily cope with the cold
So long as they have plenty of forage, they can warm themselves up,
although of course you can make it a bit easier for them by providing decent (but not OTT) rugging.
IMO, horses find it far more miserable and exhausting if they get prolonged periods of rain, like we had at some times in November, or intense winds, than it being cold like we have right now. Mine are out 24/7. The oldies has a MW/200gm rug, and the other one (leg injury, but otherwise v well) is in a LW/no fill just as a precaution, to stop her getting soaking wet. Both have ab lib forage and are v well and v happy.
I may be in a minority, but I don't worry about them at all purely cause the temp has dropped, and would feel FAR worse if they were being stabled excessively because of this weather
I heard it got down to -17 in some parts of my area last week. Thats damn cold I have to say! The lowest I've personally seen during this cold snap is -10.
I may be in a minority, but I don't worry about them at all purely cause the temp has dropped, and would feel FAR worse if they were being stabled excessively because of this weather