Beverly, I doubt that is the case. Grain changes or a sudden overfeeding can cause laminitis for any horse. That’s horse care 101. I very much doubt that Secretariat’s caretakers would have assaulted him with a “sudden” grain increase.
I suspect it is more likely that he had developed insulin resistance and no one knew it. Not much was known about IR even a few years ago. I also suspect he could no longer tolerate a diet that had served him well throughout his life, which is what happens with IR horses. When insulin resistance occurs, ANY quantity of grain or sugars can set these horses over the edge (even 1/4" cup or a lump of sugar or a carrot), but it doesn’t often occur until older age…esp. in TB’s. A bit like Type II diabetes in the elderly.
The metabolism has changed without notice. Unfortunately in stallions the changes can be subtle or unseeen … and “unexplained” laminitis is often the first indicator, instead of the the lumpy fatty deposits and fat crest that can alert us to the predisposition to laminitis and allow us to stave off a founder in other horses.
When laminitis is the FIRST indicator because the physical, body changes were not apparent, the damage is done…the management changes in diet just come too late for some to ever really recover from the devastation to the hoof.
Because stallions have superlatively muscled bodies and large neck mass anyway, I also insulin resistance is hard to suspect as they don’t fit the “look”…so unexplained founder may be the first indicator.