The AGRIS DTD, available at http://purl.org/agmes/agrisap/dtd/ was recently updated.
The changes done consist in the removal of the obligation of three core elements:
which are now optional. Note that AGRIS still welcome a rich bibliographic description of all the possible metadata fields. The change was required to allow repositories, storing data that may not include those metadata tags, to be harvested by the AGRIS database.