I thought I was missing something, but think you're referring to the 10lbs alfalfa as 'hay'?
Seriously?? There are people that think 10lb of roughage daily is adequate for a full sized horse? I disagree thoroughly. Horses need *at least* 1.5%bwt daily in roughage, but 2-3% is generally appropriate, *for maintenance*, not weight gain* So... 10lbs = 4.5kg(I'm in Oz), which is 2% for a 225kg horse - IOW, smallish pony! And if they're 'easy keepers' who get fat on that much, don't reduce amount, but choose something lower sugar/starch, or soak it before feeding!
So OP, assuming the horses get no grazing or other hay, they would need *at least* double that much hay. I also wouldn't feed alfalfa as the main forage ration, as being so high in protein & calcium, etc, can be too much for many horses & also imbalanced nutrition. I'd personally provide them with free choice grass hay + the 10lbs alfalfa.
As they're 'hard keepers', especially if they've been racehorses or otherwise intensively kept & stabled a lot, especially as they've been on high starch & low roughage diets, I'd want to treat for ulcers & feed a probiotic, to help the gut become healthier.
I can't find an ingredients list anywhere for the Nutrena, but I wouldn't feed them any grain as a rule, or other high starch/sugar ingredients, as this can actually contribute to weight and other health probs. I'd instead look for a healthier alternative such as beet pulp, rice bran, etc. If you must feed grain, ensure it's fed over as many(at least 3 if poss) small meals daily as you can, rather than just 1-2 big meals, to minimise the 'side effects'.
Well balanced nutrition is another consideration for 'hard keepers', as they may be that way due to mineral deficiency rather than lack of calories.