I've read a few posts over the past 3 years giving various opinions on this - most recently I read a post stating that it would be better for the environment if subsidies to upland sheep farmers were removed.
I can only think of one poster here who farms sheep in the uplands and maybe one or two more that might. I'm not, though have kept and worked with lowland sheep.
My tuppence to add is that upland grazing by livestock is environmentally beneficial.
Lowland sheep farmers rely on the highland breeds to X breed with to enable the lowland lambs to be hardy enough to survive the fickle UK climate. It really does matter 'how far up the mountain' your breeding ewes come from.
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