Not sure if they are poisonous or not, cows love them, but horses just don't eat them!
They need to be sprayed before they flower as once flowering they won't die! I once cut them off at ground level and painted the root with weed killer - they didn't grow back but some of the ones that I burned on the bonfire actually grew!
I've managed to greatly reduce the numbers in my paddock cutting and painting with weedkiller but some still grow each year. Most important thing is don't let them seed.
Not poisonous but as Tnavas says, neds don't usually eat them. When I was little (when dinosaurs still roamed the earth) dock leaves were supposed to be good for wasp stings so that may make them a little more alkaline (or acid - whatever is the opposite to wasp stings) and thus nasty tasting.
Has anyone ever tasted one in the interests of science? The mad people on this Forum (me included) have pretty much tasted everything that their neds eat. If anyone is interested, D&H Daily Vits & Mins is disgusting lol.