An upset in the horse's metabolic system eventually shows itself in the hoof wall texture and horizontal rings. The hoof becomes a mirror of the internal imbalances.
MMP enzyme, (matrix metalloproteinase) controls the growth and direction of the laminae in the hooves. This enzyme is well regulated in a healthy system providing flexibility in the connective tissues of the hoof wall.
The cecum, in the hind gut is part of the digestive process. It is a fermentation sac containing microbials that assist in breaking down the forage.When the diet is high in grains and sugars this causes the bacterial population of the hind gut to rapidly increase damaging the lining of the colon and releasing toxins into the bloodstream.
Eventually the toxins reach the hoof causing MMP's to be released. In efforts to process the excess toxins MMP which is normally well regulated is released in abundance. The result is a separation of laminae. This is a painful and serious condition. The results are seen as horizontal warped rings on the hoof wall. See photo below.
The next photo is of a healthy hoof. You will notice no rings, a smooth surface with the coronary band even and smooth. There is about a half inch of new growth below the coronary band. This too is healthy and smooth.
There can be other causes for horizontal hoof rings; improper trimming, shoeing, concussion, abscesses, circulation issues and other metabolic syndromes.
A diet high in sugars, grains, and starches is one of the main contributing factor to hoof growth deformities and other health issues in the horse. The horse needs a diet high in fiber. They are grass eaters, not grain eaters.