"She is eating the beef."
Translation:היא אוכלת את הבקר.
It is only necessary to indicate a definite direct object. Anything that uses "the" or is a proper name or is otherwise definite.
If you are using the whole "בשר בקר" you need the particle ה on the second noun because it is two-noun construction: בשר הבקר = beef
I wrote "היא אוכלת את בשר הבקר" on a translation exercise, and it was marked wrong. This looks like it should be reported.