ABUJA, Dec 21 (Reuters) - Nigeria’s parliament approved on Monday the government’s 2021 budget of 13.6 trillion naira ($35.66 billion), assuming 3% annual economic growth, oil prices of $40 a barrel and 1.86 million barrels a day of crude production. The budget will take effect when Nigeria’s President Muhammadu Buhari signs it into law, though a date has not been set and he could request changes. ($1 = 381.0000 naira) (Reporting by Camillus Eboh; Writing by Paul Carsten; Editing by Alex Richardson)