Are they getting ad lib hay? How old are they? What have they been used to in previous years? Can you ask the previous owner's opinion? Shetlands mostly have twins. Of all the primitives, Shetlands are the most likely to get fat, which with big cross-bred lambs would not be good. They store their fat inside around their organs, so it doesn't particularly show on a typical condition score - you can assume they are thin when they are not, but equally you can miss noticing when they are thin. We always fed our Shetlands a little 'tup and lamb' mix in the final 6 weeks up to lambing, but nothing like in the amounts a commercial breed would need, and not the 18% protein kind, but that was breeding pure, not to a larger sire. The only Shetlands I have seen looking fat next to a Texel are my two wethers, which are humungous, although they get only grass in summer, and hay in winter.