Where the boards show the most wear is where they mostly go bad, at the end of the trailer in the corners. Luckily that isn't a spot where the horses are likely to step, but it is up against all the metal in the corner. I normally replace them for that reason. You can check those spots from underneath and see the rot has worked its way through. Check to see how soft the wood feels. If you feel it needs replacement, you'll most likely have to replace the entire board, which seems to be a waste unless you have use for the rest of the board that's in good shape. What I did with my trailer is weld another angle iron against the back of the one installed at the factory. That way, I only have to replace the board up to that piece instead of the entire length of the trailer.