5PCC,
I know I'm about 4 years late to the party, but I just came across this and didn't want to see your question go unanswered.
Whole oats are oats that still have the hull on the outside (and groat inside)
Groats are oats that have been hulled (to remove the hull)
Steam crimped oats are whole oats that have been steamed, then rolled...with the hull still on
Steam rolled oats are groats that have been steamed, then rolled...no hull, and they look about the same as oatmeal you'd have for breakfast
It all depends on how you want to use them, as to what your best choice would be.
Whole oats may be good for cattle, if you want the added fiber (the hull is pretty much all fiber)
Groats are going to be difficult to come by, because few processors just hull oats and sell them as groats. However, you can sometimes find them...they'll either be "green" (unprocessed, just hulled) or "steamed" (hulled and steamed, but no further processing). Steaming breaks down the substance in the groats that would otherwise cause them to go rancid/spoil quicker.
Steam crimped oats may be good for cattle, but again, only if you want the added fiber. They are generally not as good for calves, as they are harder to digest.
Steam rolled oats are excellent for cattle, especially calves, because you get all the protein and fat, but far less fiber...and they are superior for gut health and digestibility.
If you want to know more, just ask - I know a bit about oats, because I'm the Operations Manager for an oat processing plant in Minnesota.