This will be long (but it's really short if you knew how much information is being skimmed over or not covered)
Since I'm in town with my son's laptop I don't have any of my old "material" I'll have to attempt this from memory. This is also going to be incredibly condensed and will be missing loads of additional information (that really should be included), but there's not way I'd retain it all (it's a course of study
, that includes a lot of information). However, here's some very condensed information. (and hopefully my memory is still not too far off).
Quick note for those who don't already understand. Horses cannot actually digest the long fiber that is mainstay of what nature has designed them to live on and is the healthiest thing for them to eat. They require microbes (bacteria might actually be a more accurate term....and might be what they taught....I honestly don't remember off the top of my head) that break it down and make it usable for the horse.
Corn, oats or barley all are less then 50% digested by the time they reach the cecum (beginning of the hindgut). Corn gets the prize for being worst largely because it is the most dense with regards to starch and has the highest % (over 2/3 if memory serves, but not sure by how much) that the hindgut is forced to deal with (over 2/3 if memory serves, but not sure by how much) trying to digest (carbs are suppose to be absorbed in the foregut so it's bad for them to get to the cecum and large intestines).
This results in change of the ph in the cecum.
(I generally dislike saying "bad" vs "good" microbes, since both actually serve a purpose here, but for the sake of the explanation I'll use bad and good microbes)
The load of carbs being dumped on the cecum and the resulting ph changed basically creates a breeding ground for these "bad" microbes that should only be present in significantly small numbers. At the same time is significantly reduces the amount of good microbes that the horse requires for digesting the long fiber (hay, grass, beet pulp, etc..) that make up the most crucial part of the equine diet. The resulting imbalance can lead to certain condition (acidosis comes to mind) and can even, in severe cases, bring on colic.
Yes, it's very over simplified and I've often wished I'd kept all my notes and materials (I was younger
and only saw it as something I needed to know so all I still have are some things I found interesting enough to save).
But basically, all grains are bad (they all end up doing the same thing). Corn, oats and barley just topped out the list (possibly because they are also the ones most commonly fed), but people feed a lot of things that aren't good. History, tradition, the "feed industry", etc.... have all contributed to people "killing their horses with kindness". Feeding grains is just one of a long list of those things (I won't even get started on the junk foods, like wheat brand, that many people believe are great, but are really bad). One of my favorite examples of this is Secretariat. One of the most valuable horses in US history. Was put down at 19 almost certainly because he was kept on the grain diet (lots of oats) that they are probably still feeding race horses. They never seem to catch on that there are better and healthier energy feeds for a hard working horse (that won't mess up their feet in the process).