Thanks for all the info. I manage a barn and we feed the horses one cup of oats and one cup of ground flax once per day with a vitamin supplement (in addition to free choice hay) and they're all fat and have beautiful coats, so that's why I was leaning more towards flax. However I have a friend who didn't really have any luck with flax so I was considering the BOSS again. Lux gets 5 lbs per day of Blue Seal Grow and Excel, so would giving her 1 cup of ground flax twice per day help with weight gain or would I need to feed more? And is it true it loses it's nutritional value if it's pre-ground? I board her and would have no way of grinding whole seeds right before she gets fed.
1-2 cups a day is usually plenty for weight gain. I have yet to feed a horse more, but I like to put on weight slowly.