"The man has a cow and a calf."
Translation:Bărbatul are o vacă și un vițel.
From what I've been told, there is no solid rule for gender. Maybe some soft rules like "I before E, except after C" in English that apply to many cases, but definitely not all of them. I think (although not certain) that offspring being male is one of these soft rules.
There is not rule to determine whether or not a word is masculine, feminine or neuter. For native speakers, the correct answer just sounds good and right. Also, yes, you are right, kind off. The baby of the cow is masculine "vițel" and only the his mother is feminine "vacă". We also have a word for his father which is of course masculine, that word being "taur" meaning bull. However there is another word for the sterilized male "bou" meaning ox. Hope I helped you :)