This has never been an issue before and I keep Amber's feet extremely clean. I'm not extremely knowledgeable on white line disease/white line separation (are they one in the same?), so I'd like some info.
It was too dark to take pictures this evening, but I will try for some tomorrow to add. She's not tender, and her feet otherwise look good (the farrier told me this).
Her pasture is dry except for one big, nasty puddle that will not go away near the water tub. The hose has a hole in it and whenever they're filling it, more water just gets put out. I'm thinking she's walking through this mud and packing her feet with the wet, nasty crap and that's behind this. She's a big water drinker and also likes to just play in the water for no reason, so I expect she spends some time there when she's turned out.
Another factor to be considered is the trim before this most recent one she was left a little long and as a result she was pretty long this time and she had much more flare than normal. Her fronts are worse than the hinds, but the beginnings of the separation is apparent on all four.
Like I said...she gets trimmed every 5-6 weeks, and I keep her feet very clean. She's never had this problem before either, and normally she's got great feet all around. But the puddle by the water tub and the long trim last time are the only two things I can think of. She got moved to the 'puddle pasture' in early December though, so that is a "new" situation there too - her old pasture didn't have any puddles.