All 3 of mine are pretty dang body sore. The only supplement they all get is electrolytes. This started a six weeks or so ago. I put out free choice salt and started them on a muscle mineral supplement maybe two weeks ago. Still sore. Could it be something in my pasture?  They are turned out most of the time. Louisiana. It dang sure isn't from hard work. 

In my experience body soreness is a secondary problem.  I think you're going to have something else going on the cause the soreness.

What do you feed? 

Are they UTD on shoeing? 

A bad/unbalanced shoeing job can cause body soreness as well, not saying thats the problem but I have seen it happen a few times.

lack of magneisum