^ this.
One of the effects of heavy concentrate feeding is causing temporary infertility. If you look at the semen sample there is often a lot of dead sperm in there. I wouldn’t buy a well fed tup, especially a lamb, early in the season and expect him to be fertile straight away tbh, and treat it as a bonus if he is. I’ve never had one that hadn’t come right after a couple of months of more normal nutritional management.
Of course, those very high profile ones usually manage to produce sons for the early sales the next year. I suspect DNA testing might throw a few anomalies on occasion though......
As I understand it, it has a lot to do with the amount of fat around the testicles, keeps the temperature up too high to produce viable sperm.
Was working on a place that had a few pedigree texels, with a ram that was was being fed 3 times a day when I arrived in september, getting to the trough was a struggle for him, he was thrown out with the commercial flock after he had been used with the pures, some of which were left empty, and when I eventually went to get him in I could barely catch him! Completely transformed on a regime of grass and exercise, produced some of the nicest ewe lambs I’ve seen out of good southie Cheviot gimmers.