It's a hay loader trailer (not sure on the correct English terminology, it'd be a "Ladewagen" in German), a Bucher T20K if I'm not mistaken. Basically it just scrapes hay off the ground and dumps it in the back, rather than feeding it into a baler. This one is just parked there with a (presumably custom) roof and the rear hatch open to provide shade and some shelter for the sheep, since there aren't any trees in that particular field - though the sheep all decided to huddle in the shade outside the trailer :)