Tambi, flax seed is recommended to counteract those pesky flies–which was really good news for me, because my Morgan mare is allergic to fly bites. I heard an interview with a large animal vet, and another vet who specialized in equine dermatology, and they both said, feed flax. I too live in the CA high desert!
OP, Omaline has been around forever and is not the best way to add weight. It contains corn, which will heat up a horse, and also molasses–which will also heat up a horse. (Remember the “Twinky defense”? Sugar makes people and horses act crazy.) Straight hay plus careful supplements is usually the best way to add weight to any horse. Try to avoid alfalfa hay, as it too can make a horse “high.”