hi I only just joined this site- I have north rons on the island. I give them very small amount pelleted sugarbeet and a friend only uses sugarbeet shreds- her argument is it is the MOLASSES that makes the sugarbeet high in copper, not the sugarbeet per se, so get unmolassed sugarbeet and it is probably no worse than flaked peas or barley for copper content. Same goes for ewe mix rations that have a high molasses content to improve energy levels and palatability- try to avoid.
Someone linked your post on another site, someone there commented sugarbeet was low in calcium (which is the exact opposite!). It can be hard to get real facts online but there are North Rons getting unmollased sugarbeet and are just fine- but only a very small amount of concentrate as all cereals contain some copper.
I have a Tub Ron from Denis Brinicombe- they never touch the blooming thing, but do like seaweed meal, which you can get from most horse feed merchants.