I just got 2 new horses. "Riot" the arabian and "Trigger" the quarter horse. I'm very aware that Riot needs to gain weight and she already has in the couple of weeks I've had her. My main question is about Trigger. He seems to be in good weight but his ribs are showing a bit. I'm used to quarter horses being very round with no ribs showing at all and I'm not sure exactly how to put this "rib" weight on him. Riot is on 2 flakes of orchard and 1 flake of alfalfa, a scoop of purina strategy healthy edge, and all day turnout on really good grass and Trigger is on 1 flake of orchard and 1 flake of alfalfa, 3/4 scoop purina strategy healthy edge and all day turnout on the same awesome grass (spring grass here in Washington is awesome!!) They are also both being worked 5 days a week (not very heavy right now as neither of them are in very good shape). They have both been on good worming schedules. I have pictures of both of them below, I'll try to get better ones soon, they don't like to stand still :p
Regardless of what schedule they were on for worming, I would still suggest a fecal be done as "good schedules" can still result in parasite loads.
There are so many possible underlying issues to a lack of weight that eliminating them is the best way to approach gain -- after all, if a horse is underweight due to bad teeth, a parasite load, illness, etc you will just be pouring money (and feed) down the drain until/unless the underlying issue is addressed. If/when you have established it is simply a lack of groceries, that is the time to determine a proper nutritional plan.