Some horses are reactive to soy, probably to the phyto estrogens. Those horses should stay off soy.
If your horse is IR/PPID and obese, you can do just fine with hay. If your horse is losing weight, then fats and oils can be very useful.
IMHO a horse should not be eating large amounts of whole wheat, organic or not (brewers grains are different). The best whole grain for a horse is oats, the traditional feed. Corn, wheat, and barley are higher in starch. Of course all horses do not need oats or any grain.
I don’t think that all cereal crops are dessicated with glyphosphate pre harvest, only in wet conditions. It is an extra input and an extra expense, so I am sure the farmers will skip that unless it is wet.
There have been several recent threads on COTH that have discussed the current anxiety about glyphosphate in horse feed, and concluded that the anxiety is out of proportion to the reality.
Anyhow, if you want an overview of how to construct a diet for a horse I recommend Julie Getty’s “Feed Your Horse LIke a Horse,” which has chapters on IR and EMS and PPID.
If your horse is metabolically compromised, you need to keep the total dietary carb levels below about 10 or 12%, which means basing your feeding plan on a tested low NSC hay. You also want to get a vitamin/mineral supplement into him, which you can do in a mash of hay cubes without compromising the NSC levels.
If the horse is underweight on a straight hay diet, then you can start feeding low NSC concentrates. I have heard of metabolically compromised horses being sensitive to soy, so stay away from that. It has nothing to do with whether the soy is organically grown. You also want to keep the horse off any major sources of starch (because starch is a component of NSC). The starch will be a problem whether or not the wheat, oats, corn, etc are organic or not.
You also want to keep the horse off free choice pasture.
If you are dealing with a metabolically compromised horse, the important thing to do is to get the NSC levels in his diet under control, and to get a vitmain/mineral supplement into him. Testing your hay is the most important step you can take. If you haven’t tested your hay, everything else is a wild guess.
Once you have him stabilized on low NSC hay, and getting a supplement into him (hay cube mash is fine), if he is skinny, then you can think about concentrated calories. Vegetable oil, maybe flax oil if you don’t want to feed a soy product, is good. Locally we have an excellent “cool calorie” product that is basically alfalfa meal and soy oil in very palatable soft pellets. Soy oil apparently does not contain the phytoestrogens.
Anyhow, the #1 thing for an IR etc horse is to test the hay and keep the total NSC in the diet low. Your horse will have problems on high starch grains whether or not they are GMO because it is the starch that is giving him laminitis. If your horse is sensitive to soy phytoestrogens, then he will react to whole soy meal, whether or not it is GMO.
In other words, going organic will not help the horse tolerate these foods.
Just like a diabetic person cannot walk into a whole foods store and gorge on cookies and chocolates made with “organiz cane sugar” and not get into serious trouble. On the other hand, if they follow a sensible low sugar/carb diet, they will do lots better even if it isn’t organic.