A "potrero" is any bit of farmland, preferably with appetizing grass if cattle are the intended residents, that has a surrounding fence, which is designed primarily to keep animals on the inside, and only nominally (and for legal purposes, since it marks the boundary between one man's land and that of his neighbour) intended to keep rustlers on the outside.

A valid generic alternative to 'fenced' would be 'enclosed', bearing in mind that the boundary of the potrero might (depending on the availability of construction materials) be a stockade, or a wall, for example, and it might be formed (at least in part) by a natural boundary such as a river.

That said, from what I recall of El Salvador (30 or so years ago...) the enclosing structure is most likely to be a very flimsy barbed-wire fence. Nothing remotely resembling the smart wooden, white-painted fences one sees at the paddocks in the world's posh race-courses.