I don't know about the toxicity of those plants, but is browsing on vegetation a cattle dog thing or something? My Flash (ACD mix) is a voracious grazer. He grabs mouthfuls of grass every time he's out...not to make himself sick or anything....he just really seems to enjoy eating it.
Every day during our walks around the fields he stops to graze and when I'm putting all the dogs back in the yard he's always the last one because he's over by my lilac bushes yanking out mouthfuls of the long grass that grows around there.