Sentence:  But until he had poured enough milk into the vat above the separator, I drove unmilked cows into the <WSD>bail</WSD> where he had previously milked and released one. He moved from one <WSD>bail</WSD> to the other to milk the next one I had readied. I drove each cow into the empty <WSD>bail</WSD>, chained her in, roped the outer hind leg then washed and massaged the udder and teats. 
Meaning of the word marked with <WSD> tags:  A frame to restrain a cow during milking or feeding.