1. a woman acting in a specified capacity —used in combination
2: a female partner in a marriage
Merriem-Webster defines a cow a either:
1. the mature female of cattle (genus Bos); OR
2. the mature female of various usually large animals (as an elephant, whale, or moose)
The wife and the cow are the most important females of both their worlds. The wife is the wife,and therefore a partner, and the cow is the wife's central and most important property.