I own a six-year-old Thoroughbred/Oldenburg gelding that weighs about 1,000 lb (455 kg). He’s ridden about six days a week—dressage four times weekly and conditioning work twice weekly. He is fed as much mixed hay (grass, clover, and alfalfa) as he will consume and a scoop each of a popular low-starch feed and rice bran. He receives biotin, omega-3, and joint supplements. He has a trace-mineralized salt block at his disposal, too. Does my horse need more minerals in his diet?
I plugged the information you provided into MicroSteed, Kentucky Equine Research’s state-of-the-art ration evaluation software program, to determine whether your horse requires mineral supplementation.
Because the program requires weights of each of the feeds and supplements, I had to take an educated guess as to how much of the low-starch concentrate and rice bran you are feeding (there are many different sizes of scoops) and assumed that the amounts you gave were the total for the day, not per feeding (if there is more than one feeding per day). For this evaluation to be more truly accurate, I would need exact weights of the feeds and supplements. Keep this in mind whenever you ask for help in the future.
As it stands, the combination of forage, concentrates, and supplements do an adequate job of meeting the recommended requirements for almost all of the nutrients except selenium. Because your horse is consuming less than the recommended feeding rate of the low-starch concentrate, it is not supplying all of the trace minerals that are needed by the horse. Fortunately, the other supplements are picking up the slack (i.e., copper, zinc, vitamins), except for selenium.