Have you ever noticed ripples or rings going horizontally around your horse's hoof walls? Those are stress rings and can be a sign of stressors such as a move, new living environment, dietary changes, recent illness, or metabolic issues.
You can use use the average grow formula of roughly 9-12 months to grow an entire new hoof wall to gauge how long ago the stress rings occurred. If the rings are at the top of the hoof wall they are new growth and happened in the past couple of months; if they are half way down the wall then the stressor was roughly 5 or 6 months in the past. If you are making dietary changes to address metabolic syndrome, then after implementing a low sugar diet watch the walls to be sure the rings are growing down and that the new growth from the top is growing down straight and well connected.
While stress rings are not necessarily a cause for concern, they can be a signal that something may need addressing if they are recent or if your horse is continuing to grow out new rings with their new wall growth.
In the case of laminitis rings you will tend to see the rings being wider toward the heels and lumped closer together around the front of the hoof wall.
What to do:
In some instances you may not need to do anything other than keeping them on a regular schedule with your farrier or hoof trimmer. If you know of a one time event like a colic or the horse having moved to a new location that coincides with the ring, nothing more is needed than to let it grow down.
If the rings are persisting and continuing at the line of new growth with no explained reason ask your vet or farrier if they see signs of metabolic syndrome or laminitis and consider a low sugar diet as well as mineral and omega 3 supplementation.