I started keeping my mare's tail braided, and now it's about a fourth of what it used to be (thickness wise). It's a couple inches shorter, too.
You can see the different here: http://farm4.static.flickr.com/3105/...bed7102d_o.jpg
It's even thinner now: http://farm4.static.flickr.com/3585/...e4773c05_b.jpg
You have NO idea how upset I am over this. It happened over just a few months.
I kept it loose at the top. I tightened it when I was about halfway down. I NEVER brushed it. I kept my gelding's tail braided the exact same way and got great results with him.
I had been using MTG at the same time, and the top part of her tail and her feathers have gotten a lot thicker. It's just the middle and the bottom that have thinned out.
I'm going back to doing what I did with her tail before this happened. Which was.... absolutely nothing.
Another thing that changed was I started dying her tail black with human hair dye. I only did it every few months. Do you think that could have caused it to fall out?