"Compare "Rosebud in June", which in the Span version makes no agricultural sense at all - "O to plough where the fat oxen graze", indeed."

I have always suspected that it's not 'ploughing' in the strictly agricultural sense that's being referred to in this song.

"And it's O to plough,
Where the fat oxen graze low,
And the lads and the lasses
To sheep shearing go."