How many weeks do you have in between trims? The When's and the Why's!

Everyone may have a different answer because there isn't a right or wrong one, there are just too many variables to have all horses on the same trim schedule. I have a few rules of thumbs for my own horses that I would like to share with everyone, but first of all I would like to here about your trim cycles :) This is my OTTB "Theosophy" aka "Tommy" he grows a lot of hoof really fast, he is on a 2 week schedule in the spring and my other horses can push it to 3 almost! He is very upright and grows a lot of heal which causes him to have toe cracks if he waits 4 weeks or more.

When I say trim I am only balancing the roof with my rasp. I never ever have to use nippers.