Core Income, including Drop Income, for the quarter ended June 30, 2018, was approximately $31.3 million, exceeding total dividend payments to stockholders for the quarter of $28.4 million. Core Income, including Drop Income, is a non-GAAP measure and is defined as net income excluding impairment losses, gains or losses on sales of securities and early termination of derivatives, unrealized gains or losses on derivatives and certain non-recurring expenses, plus Drop Income (as defined below). Core Income may differ from GAAP net income, which includes the unrealized gains or losses of the Company's derivative instruments and the gains or losses on Agency, Credit Risk and Non-Agency and Interest-only Securities.
As of March 31, the Company's Agency Securities portfolio consisted of Fannie Mae, Freddie Mac and Ginnie Mae mortgage securities, substantially all of which are fixed rate securities, and was valued at $6.2 billion on a trade date basis. The Company's Credit Risk and Non-Agency Securities portfolio was valued at $1.0 billion, U.S. Treasury Securities valued at $0.7 billion and the Company's TBA Agency Securities valued at $4.3 billion. During Q1 2018, the annualized yield on the Company's MBS portfolio (including TBA Agencywas 3.05%, and the annualized cost of funds on average liabilities (including realized cost of hedges) was 1.53%, resulting in a net interest spread of 1.52% for Q1 2018.