Austen asked about keeping weight on a hard keeper, while keeping sugars and starches to a minimum.
In the UK we find linseed to be a hugely valuable feed. It is high in top quality protein (all the essential amino acids) and omega 3 and 6. It is also a useful source of copper. As a bonus when wetted it produces mucilage which is very good for the gut. And it doesn't make horses fizzy. The down side is it smells gorgeous, tastes nice and the horses get somewhat addicted to it.
I have also found it is a good product for hiding nasties like bute, wormer and sedalin.
If you want shiny hair and great skin try it - and let your horse have some too!
For an example of the results see this horse - just after she completed a 25 mile endurance ride.
There are two ways to feed it; micronised is the best, but not always easy to source. In the UK I only know of one producer Charnwood Milling. If you can't get micronised you will have to cook your own. This is an art and you must be patient to get it right. I used two ways:
1) Huge pan (couple of gallons would be ideal) 4 pints of water, 2 egg cups of linseed.
Bring to the boil - watch carefully because linseed is notorious for boiling over and because its high in oil it is very very hot. Then keep at a fast simmer for 30 minutes. Watch it very carefully and add more boiling water if you need. You will end up with something between a thick snotty slime and a jelly. I used to add mint as it cooled because my old horse loved it.
This process does destroy the quality of the oil but not the calorific content.
2) Microwave. Similar to above, but much less stirring and fretting required. Also with microwave safe plastic the washing up is easier. Bit of a pain if you let it boil over though.
Whatever you do, don't use the linseed lozenges. They are a by-product of the linseed oil industry. The valuable oil is extracted (to be wasted on wooden floors) and the left over mass is glued back together using molasses and sugar syrup. The resultant lozenges were originally intended for the cattle industry - but typically some bright spark thought they could turn some extra coin by selling them to horse owners.
For an average keeper, but working hard, 500kg horse, I used to feed one egg cup of the seed (which was then cooked as above). Or one 1/2 pt mug of the micronised version.
For hard keepers you can up the quantities. Linseed is very low in sugar and starch so I never worry about feeding it. But of course there are always exceptions so introduce gradually and see how you go.