While we were at Auburn [Clinic Report], we asked an ag instructor about Milton. She was unconvinced on soy as the cause [Clean Cups!]. She asked for scans of the feed tags. In the event anyone else would like to float a theory, here is the nutrition information:
Dr. Jekyll
Manufacturer’s page: Purina Omolene 100
Mr. Hyde
Manufacturer’s page: Triple Crown Senior
Virtual Brush Box 
If it works, don’t mess with it!
No desire to mess with! Would be nice to understand why.
I would actually have to see the feed to comment more accurately … but from the feed tags, I can at least tell you that the Omelene is a much lower energy mix. Not sure why you would feed a senior feed to a TB under 10 … senior feeds are designed to deliver dense calories and adequate protein to horses whose ability to digest efficiently has been compromised by age (basically, they overdo it because they know senior horses won’t use a good percentage of the ingredients). There is probably more soy in the senior mix — it’s a good way to deliver that 14% protein — but your ag instructor is correct in that pinpointing soy sensitivity is a stretch based on the info you currently have. You’d need allergy testing to be sure, and you’d probably find that if he is sensitive to soy, he’ll also react to alfalfa, clover, and other legumes as well. The Omelene contains some soy as well, so if he’s handling that, it’s probably not the root cause. It’s possible that the senior feed just delivered more energy than he knew what to do with — and since he wasn’t being ridden or asked to do much, it translated into crankiness. (With some horses it would have translated into nervous energy, running the fenceline … with others, simple weight gain. Depends on temperament.)
My suggestion? Minimal grain. Lots of soaked beet pulp. Until he is actually, you know, working, he just needs to keep his ribs covered, he doesn’t need much energy feed at all.
Could be. OTOH, it seemed to be more than calorie overload. The whole thing was deeply, deeply weird. More on the subject tomorrow (Monday 25).