I would say no, it's not a universal truth.
It *was* certainly true for dogs like
Stormbringer and
Pretty Boy ...
But it was NOT true for dogs like
Thunder and
Duke Nukem ...
Thunder's sperm was tested at 11 years old ... and had a 600,000 sperm count ... while Stormbringer was shooting blanks by 5 years of age.
Thunder was a well-kept house dog and could produce till the day he died at 12 years of age ... while Stormbringer was fought/rolled constantly before I had him, and had infections and other problems ongoing all the time (so was that genetic or circumstantial, I don't know?).
I would say any of my dogs (or any dogs down from Hollingsworth stock) that are fed kibble, and fought often, will have a VERY SHORT breeding career.
Their fertility *IS* severely affected by corn-based foods ... which is why I tell so many BOZOS **not** to feed my dogs kibble (but who listens?).
It has also been my experience, that any of my dogs fed raw (like *I* say to feed them, not some incomplete spin off it), and allowed to heal fully, will have a completely normal and full stud career
Jack
PS: Of the dogs above, I would say only Pretty Boy had truly genetic infertility, as he always had BBs for balls ...
The other dogs were ALL normal (except Stormy) ... and he was perfectly fertile early on ... but was fed shit food most of his life, and was constantly being banged-up, too often and too much during his life, by Leon. He always had a constant tooth infection from gum damage, and THAT (more than genetics) is why his breeding career stopped short.