The beef is a source.

"The beef is a source" would be extrememly confusing. Even "The beef is a source of meat" is awkward. Beef is not "a source of meat." Beef is meat. You could say "Cattle are a source of meat" or "the cow is a source of meat." But just saying "the cow is a source" would be very strange. I'm really not sure what you are trying to say.

Regardless that it is strange, we can still say "the cow is a source" right?

You would need to say "The Beef is a source of meat.", but this is not quite right: beef is a type of meat, not a source of meat.

You could say "Cattle are a source of meat".

"Cattle are/Beef is a source." is unlikely but theoretically possible when the thing it's a source of has just been mentioned. For example:

You need to make sure you get

Isn't saying "Cattle is a source." instead of "Cattle is a source of meat." exactly like saying "Having *** is an act." instead of "Having *** is an act of passion."?

Why can't we merely say "Cattle is a source"?????

A source has to be a source of something. As I mentioned, if that something is apparent from context then "X is/are a source." is possible. This tends to sound a bit abrupt, though, so it's more usual to say what it's a source of in the same sentence, either explicitly or using a pronoun such as "this".

It should be "Cattle are..."

Incidentally, "Having *** is an ac

What? Can't we say either cattle are or cattle is?

"Cattle" is plural.

Ok, so we can't say the beef are a source of meat?

wholegrainOk, so we can't say the beef are a source of meat?

"Beef" (in this sense) is uncountable, so it should be "beef is".

"the beef is" is also possible, but only if you are talking about a particular batch or type of beef.

We've already established that "Beef is a source of meat" is not a very sensible thing to say.

I meant cow. We can only say "cow is a source of meat" right?