They are the cattle of the field, and we sit down to eat beef. What's the meaning

This is a joke. The joke is as follows:

A man went to the doctor with injuries all over his body. The doctor asked: "How did you get into this?" The man said: "You can drink milk." The doctor was surprised: "You can drink milk. Is it like this?" The man cried and replied: "While I was drinking milk, the cow suddenly sat down!" As the saying goes, "The old cow paid off his farming debt and lay on his head in the sunset." Iron began to appear in our country during the Spring and Autumn Period and the Warring States Period. Plowing with oxen, as an important production tool, oxen occupies an important position in our country's agriculture. Even today with such advanced mechanization, oxen are still used in many rural areas, and there is a popular saying, "A kneeling old cow cannot be killed." ”