Results

 Sophie's blood work came back and with me being a "they really just need plenty of good hay and an appropriate amount of ration balancer" person,  I learned a few things.

Here I am sharing in case it helps someone else out.

Sophie's Vit E levels came back very low, and zinc levels were on the low side. As per the vet this means:  

 "Vitamin E is a powerful antioxidant, essential in tissue/skin/fur/foot growth and repair, normal nerve and immune function. It is plentiful in fresh grass but degrades very rapidly in all forms of cut/dried forages, therefore almost always requires supplementation in horses since unlike other vitamins, they cannot make it themselves. She is also low in zinc, which also plays an important role alongside vitamin E for normal growth/repair and function. Deficiencies in Vitamin E and/or Zinc commonly present themselves as poor/abnormal coat and hoof growth, neurological deficits and/or muscle weakness."

Well, that all adds up, doesn't it? 

Two of my big questions: "Why is she not gaining more weight or topline given the amount of food she is consuming?" "Why is her winter coat so excessive?" 

I was previously adding a 'complete' pelleted feed with her hay, as well as a vit/min supplement at the high end of the recommended intake for a horse her size. On the vet's suggestion, a few weeks ago I substituted a ration balancer for those two items as it's higher protein/fat. We've upped the ration balancer a little further to account for the zinc needed.

That leaves the vitamin E. The best place for horses to get vitamin E is in fresh, green, spring pasture. That's a difficult thing to do mid winter. 

Hay can provide some, but the vitamin content goes down drastically as the hay ages, so by this time of year your spring cut hay is very likely not providing much at all. If your hay was a later cut or the quality wasn't great to start with, you definitely need another source of Vit E.

Googling tells me that the ration balancers and generic vitamin/mineral supplements available to me here don't have the amounts needed to overcome a deficit, they're more on the minimum baseline level. 

I'm reading that in the wild, horses would have a surplus coming into winter, and gradually use those reserves until spring, so it's somewhat normal for amounts to fluctuate through the year. Many horses have low amounts at any given time but don't show any clinical symptoms.

I think I hit the worst case scenario - we had a drought all summer so the pasture wasn't overly rich, my hay was also harvested in similar conditions, and then we moved and Sophie lost a bit of weight, so came off fall pasture a bit underweight. She'd likely had already used up whatever she had acquired in the spring. With normal vitamin mixes just providing a minimum healthy amount, we weren't ever going to catch up and no matter how much forage I feed it would have never been adequate.

Not the best pasture even in the best of times - I was thinking it's perfect for ponies, but I'm now aware I can't rely on this type of pasture to provide enough for S without keeping her on the higher end of supplementation.

So, that's where we're at. We're going to give a vet prescribed Vit E supplement a go for a couple of months and redo her bloodwork. There is still a good possibility there is more going on affecting her weight and ability to process her food effectively, so unless there is a drastic change we'll likely proceed with scoping for ulcers at that point as well.

Here's a link to an article that I thought provided a helpful overview of vitamin E in a horse's diet, even though some of the references to supply chain issues are likely not relevant anymore.