St. Louis took Pittsburgh defensive tackle Aaron Donald in the first round and running back Tre Mason in the third round.
Richardson was a seventh-round pick in 2013 and rushed for 475 yards with a 4.8-yard average as the change-of-pace backup behind Steven Jackson as a rookie. He entered training camp as the starter last season but was hampered by foot injuries while rookie Zac Stacy emerged, falling just shy of 1,000 yards.
Cudjo had been second on the depth chart at right tackle behind Michael Brockers. He made the team as an undrafted rookie in 2010 and appeared in 13 games last season.