Is it true that a hoof that shows signs of trauma will have rings on it, either from an injury, founder, or etc...? Someone told me that the rings are like looking at the rings of tree, it can give you an approx age of the injury.
It is kind of true. Hooves will show stress rings. These rings may be simply from a sudden change in feed, ie going onto grass, changing grain etc. or it can be a sign of past laminitis. It takes a hoof up to a year to grow completely new horn, so yes it can show approximately when. It can also be an illness related thing. Basically it shows stressful times to the horses system.
I can't say it isn't completely true. I know if you were to, say, start your horse on a hoof supplement, you will see a line where the new hoof is growing in. It takes about a year for the hoof to grow all the way out. When an abscess blows out, it leaves a horizontal line, yes? And that line will continue to move downward as the hoof grows, but will eventually disappear. So I would think you can only keep track of a hoof injury for the year it takes for it grow.