I own a 9-year-old, 1250-pound (566-kilogram) warmblood-cross gelding that I ride five times a week, typically jumping and dressage work. He is a spooky horse by nature, but I am wondering if some of the unsettled behavior could be due to his diet. He eats four pounds (1.8 kg) of a KER-formulated feed that includes corn, as well as timothy/orchard grass hay. Could the corn be causing the behavioral issues?
Some horse owners believe that corn does cause excitability or unnecessary exuberance in certain horses. The feed manufacturer that you're currently working with also markets a high-fat, high-fiber feed that contains no cereal grains. I would suggest switching your gelding to this no-grain feed and monitoring his behavior for several weeks. The difference in energy levels between these two feeds is minuscule, so your horse should not experience any changes in weight, provided you do not alter his forage intake.
Cereal grains provide starch to horses, and certain horses will lose a significant amount of energy when starch consumption is minimized. If your gelding becomes too sluggish under saddle, especially for eventing work, you can switch back to the original feed or add a small amount of feed that contains cereal grains.
The amount of hay you're feeding seems to be spot-on. Be sure to give him access to a salt block.