Wow thanks for the update and in depth response! I also couldn't narrow it down, all I found was that red yucca isn't actually in the yucca family. Going to keep looking, and put something around the plant for the time being. there's never a shortage of food, when he's hungry he has a large portion of his enclosure covered in both naturally growing weeds and a grazing patch we grew with all safe seeds, plus he gets regularly fed romaine, red leaf, etc. Thank you again!I usually type something like “california desert tortoise <plant name> and see what happens. This one was difficult because (a) there is a town named Yucca Valley; (b) the word “red” is everywhere when people talk about the desert and finally (c) another name for this yucca is Chaparral Yucca and chaparral is half of the desert’s terrain. Really hard to narrow down something with multiple common words.
I actually think the Latin name—Yucca whipplei spp—is listed as a desert tortoise food on a tortoise food sheet that won’t load, probably a dead link. Naturally the study was produced in... Yucca Valley.
You can also just search for “Arizona or California desert tortoise native diet” to find sites with food lists.