Mayfield was optioned to the Astros' alternate training site Thursday, Julia Morales of AT&T SportsNet Southwest reports.
Mayfield entered the starting lineup just once during his time on the major-league roster, but he'll lose his spot in the majors as the active roster goes down to 28 players. The 29-year-old went 1-for-4 with two strikeouts and one run in two appearances with the Astros to begin the season.