REPLY: I think you have to do what you feel is right for your own horse, in the end. I think horses this particular year/winter, at least where I live in Southwest Virginia, got an unusually late start into winter. Our temps here were pretty mild for winter UNTIL this past week/mid February, and suddenly winter came with a vengeance. Temps dropped suddenly. Snow here & there. And the horses' coats weren't as thick as they usually are by this time. Only one of my 4 horses started to shiver and since I don't blanket, but work with them to build a faster coat, I started supplement feeding that one horse A LOT. And I mean as much as he'll eat of the supplement in one sitting when/if shivering (I use Triple Crown Complete pellet feed from Southern States, with Grand Complete supplement thrown in). And served as a hot mash (lots of hot water soaked in--this also helps more water to get into them so they don't get dehydrated). He ate and ate and ate it in one sitting voraciously (I think it was about 4 scoops in the end that time) and he settled down, stopped shivering, and never did shiver again subsequent days. I still supplement that every day through the colder weather (couple of scoops for him in particular, a third if he's voracious still at the end of the 2nd scoop), as well as we keep hay out everywhere here for 24/7 access. And ice scooped out of their water troughs daily.

That's the trick with horses who suddenly start to shiver at the first onset of first cold spells of the season. Feed them up! If you keep feeding that hot mash, they will stop shivering and it kicks in their coat growing faster. Even overnight often! Here's a link on my web site that talks a little more about this/warming up a horse naturally:

Cold/Chilled Horse - How to warm up horse when winter chilled

This only happened this year with one of my 4 horses here (for the first time), and it was my own new horse, Sundance, that I just bought this past fall. And I can guess why, too. He's probably been blanketed in past winters (by previous owners perhaps?) and that's the cycle problem it starts. Let me explain. I was sent the following recently, and I think it explains it all very well & I'd like to share it with you now:

When cold, a horse can, through muscular action, naturally raise the hairs on the skin, creating a thermal blanket to protect itself. When hot, the horse can, through muscular action, naturally dilate blood vessels near the surface of the skin to cool off. Additionally, it can raise the hairs and even point them in the direction of a breeze to cool down!

If you put a rug or a blanket over a horse for any length of time, then, like all muscles, after a while those muscles will atrophy, making it impossible for the horse to raise or lower the hair on its skin naturally. In such circumstances, a blanketed horse is then stripped of its only protection when urged to go out on a crisp, cold day to work. It no longer has the capacity to warm itself and the core body temperature is lowered. Or a horse blanketed in warm weather is then left to sweat and be unable to cool off normally. Blankets are inefficient in terms of heating a horse, since it leaves the belly and upper legs exposed to the cold. A blanket or rug, however used, effectively robs a horse of its natural, vital, efficient thermo-regulatory system. It also prevents the horse from receiving the benefits of a good roll on the ground, getting dirt on the skin and the hair which are natural methods of cleaning from sweat and grime, as well as protection from flies and other insects.

Well said! So, when you blanket at first onset of cold, this starts the cycle of the horse not able to warm itself naturally and those muscles even atrophying. Better to work with the horse, feeding them up (hot mash), and most all horses stop shivering at that point, and this allows the coat the spurt it needed to grow more fur.

A couple of my other 4 horses like to roll in mud as their winter coat first comes in, and this is a horse's natural way to keep warm temporarily. I don't interfere with that. Usually, when well fed up at first cold spells, horses will grow that extra-needed winter coat pretty fast. And when I say well fed up, I don't mean just hay -- I mean: adding a sufficient dose of  a supplement "complete feed" into their diet to speed things along there.

Within only a couple of days, following that above route, my new horse Sundance's coat growth kicked in finally. And he's far more comfortable now. And no worse the wear for being a couple of days behind our other horses there, winter-coat wise. And his body is learning how to grow that needed winter fur faster for his life with us here now. I don't know if he was blanketed in the past, am just guessing he was, because I see this a lot/that problem caused by blanketing. Or it might just be Sundance's own individual metabolism. Time/more years with me in the future will tell.

Now...all that said...I still say: do what you feel is best for your own individual horse. Only you know your horse better than anyone else. If I had a horse that I could not get to stop shivering via the above route (or a severely underweight horse heads into winter still underweight, which is quite common with newly rescued horses), of course I would blanket. But I've just never had to do that before. But I'm not saying I never would if the situation called for it. I would! You have to put the horse first, over "principle" and do what's best for them in the end. But try the natural route first, like I've described above. She should be kicking in her thicker coat shortly, now that her body is convinced winter is indeed here now to stay. I think this freaky winter we've had this year threw off a lot of horses, but getting pro-active there, you can encourage the coat growth along, while watching them closely.

I see so few horses incapable of growing a sufficient winter coat IF they are not interfered with there via blanketing. But I think you're doing everything right there, to be quite honest. You're watching closely, you're stepping in and helping when you see she's not quite there yet. Keep it up, you've got her best interests in mind. If/when you see her shivering, that's your cue to bring out the supplements, make a hot mash out if it and feed her up! She'll let you know there when it's enough. And don't really worry about overfeeding her in that kind of circumstance. Feeds like Triple Crown Complete (or Purina Equine Feeds, there are many more excellent ones) are complete feeds and designed for that. Feeding that (especially in a hot mash) WILL stop the shivering if you feed enough there. And for a couple of days you might need to do that twice a day. Watch her and see how she does going this route and adjust as needed.

You're a great horse owner, by the way! Your horses are so lucky to have you in their lives! And I really mean that.