I don't believe there is a cactus that is poisonous to torts. If it has spines on it, you may want to remove them. Other than that, I would take a piece and see if your tort will eat it. BTW, cactus is the only plant that I would do this with. To double check, you may want to id the cactus plant you have and then do a search on the net to see if it is edible. If humans can eat it, more than likely, animals can too.
I know some succulents in general can be poisonous but cacti are generally safe. If I run out of food for my baby in the winter sometimes I sneak a few leaves off my mom's Christmas cactus... don't tell my mom though, she'd be unhappy about that, haha.
That is most definitely trichocereus pachanoi aka San Pedro cactus. I have over 50 on my property. The skin contains mescaline! It is used as a hallucinogen by shamans in the Andes. I wouldn't suggest giving it to your tortoise!
it might not be a san pedro cactus...there are cacti that look just like that that are non psychedelic, but the members on here are right: not all cactus is safe to feed. The cactus that I have which looks exactly like that produces edible fruit: I eat this fruit sometimes, so if your cactus flowers and produces fruit, it might be safe for a redfoot to eat...I'd avoid non opuntia species for wholesale feeding, though, just to be safe