Re size of your flock - it all depends on the quality of your grass, if it is sub-divided or one large field (always better to have a number of smaller fields - you can keep one clean for lambing for example), if you are lambing outdoors or inside (Shetlands if lambed in mid-late spring are very happy to lamb outside) etc etc. But it is always better to start small and top your field a couple of times in summer if they don't eat the grass down, then having to buy in masses of hay in winter, having an overstocking problem because you cannot sell your lambs/hogs etc etc. (ask me how I know...
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Re tup - again depends if you can separate him after he has done his thing. I have left tups in with the girls until very close to lambing but they will need to be away during lambing and when the ewes are with young lambs preferably all summer. If you haven't got any separate field (normal stock fencing will usually be enough as long as he has castrated wethers in for company), then buying in a tup lamb in autumn and eating him in late winter (when the tup taint has worn off) may be the best option. I wouldn't worry too much about keeping your own tup for any length of time, there are always plenty for sale in late summer/autumn. Some breeders may also give you the option of tup hire.
My most recent tup became very aggressive when the ewes were in late pregnancy and got a little feed. Not just towards me, but towards them as well, to the extent that he was even not letting them get to the hayrack. So there was only one solution
, and coloured Shetlands do not make much money in the cull market (and I cannot afford to take a single animal to the abattoir 70 or so miles away) It was also too warm (and I was too busy) to do a homekill.