Try putting this in your DB Backup script:

delete volhist type=dbbackup todate=-28

This should delete your database volumes older than 28 days.
It's also worth comparing how long you keep your database backups to your
storage pool volume reuse parameter. You may well find that if you restore a
28 day old database backup your tapes might not be consistent if reuse delay
is less than 28.

cheers

Tony Phipps
ADSM sufferer.