The pic only comes up small for me, so can't see enough detail to say what it is. Tho it doesn't look like 'just new hoof growth' to me and is not IME anything like a line that happens from diet change. If your horse has been in constant wet, the periople layer at the coronet can turn white & be a bit thicker. It doesn't look quite like that's the case though, looks lower too. I'd probably just wipe them over with some t-tree oil or such, which should take care of any superficial infection.
My horse had these at the old boarding place - it was because of the marshy ground in the pastures, and also, sadly, in the stable. However, we discovered also thrush in all four of his hooves (about which I'm still angry to his previous owner), and I read somewhere that this white line can be connected to fungus, so maybe it might be a good idea to ask this to your farrier when you next meet him.