Like Sabrina, my horses will eat them once the sting has died down later in the year. When I find the time, I do cut nettles, and dry them on the fence in the sun for for a few days, and the goats like them once dried.
That weedkiller, my neighbour uses it in her field, and its is effective, but forgotten the name. I am pretty sure she keeps her livestock off though for a couple of weeks after spraying with it. I would imagine, this is because goats etc. would probably eat the wilted weeds, and they would be covered in the week killer.
Being organic, we do not use this, but obviously have to put up with nettles and dock leaves!!