I just had the vet out for this - my 32 yr.old just started and my 33 does it occasionally on hay. The younger guy is missing a BUNCH of teeth and the ones he has have just lost most grinding capabilities. The older guy has all of his teeth, but they are pretty useless for grinding. Just old guy teeth. I was told to keep them on pasture as much as possible, (neither quids grass, yet), keep up with their soaked senior chow, and keep giving them hay to chew, knowing most will be wasted, but they will chew and suck the liquid out. Also, keeping a close eye on weight and condition as time goes on, knowing they may need an extra daily meal of soaked senior chow in the future.