We sent our entire boys at 18 months old, and the feedback has been very positive - lovely flavour and lean.
Our vet recons the meat is leaner if you leave them entire.
A well respected breeder we know also recons that if the ram hasn't "pleasured" a ewe then there will be no taint as the hormones have not entered the body?
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I think it's to do with when you kill them, I think it would need to be out-of-season so to speak, so when the majority of the ewes are not in season. Feb-Aug I would say.
I've just got back an 18 month old entire BWM ram that has been hanging at the butchers for a week. I was worried that he would be too fat as he was very chunky, although I could still feel his spine etc. He's actually come out quite nicely, and he tastes lush. If I had more land I think I would run more on for hogget.
(actually this ram was a short scrotum castrate, but in reality he's still got his balls so has all his hormones etc)
I think it is true that wethers put on fat more easily, I certainly have seen this in mine.