If anyone is having trouble fattening their horse I would suggest this, it worked for me and I guarantee it will work for you.
Equal parts of :-
My horse was an RSPCA case and we could not find any other foods or combinations that would work for him, but then we came across this and he put on just over 75kg (165lb) in less than a month! You must wet the mix down completely and make sure you leave it soak for about 5 mins so that the copra is all wet and soaking. You may use steam flake barlie or you can boil it your self. The lupins are the same you can buy already cracked lupins or you can boil them yourself, with the barlie I didnt really find much difference but boiling or soaking the lupins overnight helped him gain weight faster.
You should be able to get all of these foods at your local stock feed or rural livestock feed sheds. Happy fattenings.
P.S you generally only have to feed them once a day but by all means if you can be there to split it up into as many feeds as possible that will put the weight on even faster.