Sentence:  The last two rounds must be ploughed shallower, and on the last <WSD>bout</WSD> the strip left should be one furrow width for a two-furrow plough, two for a three-furrow, and so on. ... 
Meaning of the word marked with <WSD> tags:  The going and returning of a plough, or other implement used to mark the ground and create a headland, across a field.