this may be helpful::
"First, the rings themselves do not appear to cause the horse any harm. They are indicators of changes that have occurred during the normal routine of its life.
They are sometimes called stress rings or stress indicators because one of the reasons they appear is when the horse experiences a period of increased stress in its life.
Hoof rings may result from something traumatic such as a serious illness or injury or something as simple as a seasonal change in diet. Moving a horse from one pasture to another or from one stable to another is sometimes all it takes for rings to form.
The only time I really worry about hoof rings is when the ends of the rings drop down uniformly a considerable distance as they come around the hoof and reach the heels. Laminitis and/or founder will sometimes produce rings that drop down at the heels.
Quite often, you can determine the cause of the rings by figuring out when they would have first formed. Assuming it takes approximately one year for a hoof to replace itself, then if the rings appear midway down a recently trimmed hoof, a check of your records from six months ago may give an indication of a change in your horse’s routine.
There is nothing you can do to make them go away once they appear other than to use sandpaper or a sanding block to smooth them out. However, I do not recommend this as they cause the horse no harm and I don’t think using an abrasive on any hoof is a good idea. You are bound to remove some of the natural protective covering that Mother Nature has provided and as the saying goes, “It’s not nice (or wise) to fool with Mother Nature.”
Therefore, it is good that you noticed them and while they are definitely something to look for during a pre-purchase inspection, they are a very common occurrence and your farrier should be able to tell you if they represent anything serious."