Not necessarily, it seems that cows are just drawn to old ash piles. When we burn brushpiles, the cows spend the time before we light them , rubbing and pushing on them. When they're burned down they seem to like the ashes, for that matter, when the flames get low they like to wander around the area. I'm always shooing them away. Being as stupid as they are I'm aways afraid one of them will walk into the fire.
As long as you have a blanced mineral available it's probably just curiosity.
burnt a heap of timber in the padock next morning cows were standing around ash looked like they were eating it does that mean there's something lacking in there diet?