The valve is selectively opened to allow a measured amount of {TAIL ~ feed material}, for example water, to fill {HEAD ~ the trough}, from which the animal is then free to drink 
The valve is selectively opened to allow a measured amount of { TAIL ~ feed material } , for example water , to fill { HEAD ~ the trough } , from which the animal is then free to drink
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH