If there are outstanding bonds of an irrigation district proposing to merge, or if such a district shall have entered into a contract with the United States, as provided in section 46-126 or 46-156, then the board shall notify the holders of such outstanding bonds that a petition for merger has been filed and the holders of such outstanding bonds may give their assent in writing to the effect that they severally consent to any merger that may be approved by the district and in case of such contract with the United States, the Secretary of the Interior shall be notified that a merger of such district is proposed and the Secretary of the Interior may assent to such merger. The assents shall be filed with the boards of directors of the districts proposed to be merged and shall be recorded in the minutes of each board and such minutes, or a certified copy thereof, shall be admissible in evidence with the same effect as an assent, but if such assent of the bondholders and, in the case of contract with the United States, such assent of the Secretary of the Interior is not filed, the board shall deny and dismiss the petition for merger.