Whole Sweet potatoes are dangerous, and you won't see with your eye the toxin that will kill the cattle. Its produced by a mold that lives in damaged parts of the skin. And its not a VERY moldy issue that you can easily see.
I know lots of people that are feeding cull sweet potatoes and are doing fine-- but all it takes is one time to wipe you out. Too dangerous for me. It would be better if you didn't have to cut them up to feed them(damages the skin) but you do.
Think of them like feeding corn with 3-4 times the moisture. So where you would feed 1 lb of corn you would feed 3-4lbs of potatoes. They are hot and can cause acidosis just like corn.
I don't know about cull potatoes but I do know that sweet potato slurry will eat the enamel off of cows teeth over time.
NC State(Matt Poore) has a good article about feeding sweet potatoes.You might be able to find it online.