Why are we certain that feed [Comparison] was the reason Milton was NQR [Cups!]? The improvement could have come from many avenues: different hay, warmer weather, a change in routine of which we aren’t even aware. So why are we so sure? Because we screwed up. I mean, beyond the baseline error of misfeeding the poor horse for months.
One night, the dinners got swapped. Two days later, Milton was as cranky and bumpy as he had been last summer. Two days after that, poof, all better. We watched the effects of that one meal pass through his system.
We are in the process of getting the senior feed out of the field. Sweet feed is not the ideal choice for Rodney, given his history of gastric issues [Aaaah]. However, the risk to Rodney is for a possible problem over time. If Milton gets any of Rodney’s current feed, the consequences are drastic and immediate.
So, Rodney does not have the most sensitive digestive system in the herd. Didn’t see that coming.
Virtual Brush Box 
It’s always tough to pin down the source of a problem in the Real World — because outside of a double-blind, controlled test study scenario, it’s virtually impossible to isolate the variables. Something’s bugging the inarticulate creature who is trying to communicate his irritation … now, guess the cause(s) and win this beautiful lounge suite! (Or, alternatively, a cooperative, happy horse who does your bidding and is pleasant to be around.) Still don’t think you have enough info to place the blame on a particular feed ingredient, but you probably have enough to say, “Whatever the reason, this feed does not agree with this horse.” And that’s all you really need to know, particularly if you have found a different feed which works better.