I usually take proverbs to be straightforward about earthly affairs. They often fall under the theme of Ecclesiastes:
A man can do nothing better than to eat and drink and find satisfaction in his work. This too, I see, is from the hand of God (Ecclesiastes 2:24)
This proverb might be a word of wisdom to farmers, touching on laziness or reluctance to work in the grime of hard labor. A framer might not like working hard with oxen because of the mess they make in the barn. They would prefer the manger to be’ empty’ as indicating ‘cleanliness’ (or’ empty’ could signify poverty). I lean to the cleanliness aspect of the barn resulting in poverty from the lack of harvest.
Accordingly, not only having to feed oxen and getting dirty oneself, but all the dung and waste that they would have had to clean up. However, to really accept the work that we have been assigned by the ‘sweat of out brow’ and the kindness that God provisioned in man’s use of stronger animals, man can reap a good harvest.
A farmer, who just wants a tidy place to relax in, will have nothing but poverty.