"The farmers count the fields."
Translation:Agricolae agros numerant.
Agri in the plural would work for the nominative (the subject) or the vocative case. Since 'the fields' is our direct object (the things being counted), we have to use the accusative plural (agros).
To show this sentence using 'the fields' in the nominative:
Agri agricolas numerant -> "The fields count the farmers" (I know it doesn't make much sense, but it is an example).
Farmer (agricola) is not feminine, it is masculine but is first declension (that's why it has the a, ae, etc. endings). While most nouns in the first declension are feminine, there are some that are masculine, the ones I can think of are professions (like considered male professions at the time) such as nauta (sailor), agricola, and poeta (poet).
I am unsure if there is a distinct term for female farmers. Someone else may know.