From time to time, we're to make a landing tread wider than 5-1/4". These requests can range anywhere from someone needing a 6" wide landing tread all the way to a 10" wide landing tread.  Regardless of the width needed, they're usually covering up the space leading to some existing flooring or used at the top of a staircase.  One customer recently asked if our Red Oak Unfinished Landing Tread 48 in x 1 in x 5 1/4 In is available in a wider tread then 5 1/4 in. for example.

The short answer is no, 5-1/4" is the widest we can make a landing tread, but you could potentially rip a retro tread with a table saw to the width you need since it is the same profile, but without the groove.  Here is a link to the retro tread in 48":

http://www.stair-treads.com/retro-treads-and-risers/replacement-stair-treads/closed-box-treads/red-oak-unfinished-retro-stair-tread-closed-48-in.html

If a groove is truly needed, you could use a router and cut the channel in the back after the desired width is ripped.