First check worming and teeth. Second ensure feeding adlib good quality hay/haylage - this means that if horse is stabled it should have some available at all times and if stabled overnight should have some left in the morning. If turned out on poor grazing then should have hay/haylage available in field if at all possible.
Bucket feeds should be based on high calorie fibre plus oil - my personal preference is for AlfaBeet with added veg oil, or you could feed Alfa A oil, or some other alfafa or grass based fibre with an oil-based feed such as Outshine or Equijewel. If that is not enough then add either micronised barley or a conditioning cube or mix.
Bucket feeds should be split into two or preferably three a day.
I have a poor eater who has been a nightmare to keep weight on. I have found after trying all sorts that the feed he likes and puts on weight is top spec conditioning flakes. It is expensive tho.
My 37 year old 7/8ths TB is on fibrebeet, Alpha A, boiled barley and corn oil, with biotal equine gold, seaweed and her joint stuff.....when I got my half starved ex racer I whacked him on the same stuff and the change was amazing, he's holding weight well too, with a good quality ad lib hay in the daytime and a net of hay and a net of haylage overnight....
something along the lines of am scoop condition nuts, chaff and 1 sugar beet plus 1 cup full of BOSS evening, 2 condition nuts, 1 sugar beet, 1 chaff or alfa a and 2 cups of boss. Ad lib haylage or good quality hay, i have always owned tb so no where your coming from.
my tb ex racer went from being lean and lacking muscle tone to very round and podgy in the last few months.i tried all the conditionding cubes and none seemed to work. now he is on alfa beet, pink powder, spillers conditioning chaff and linseed and he is looking great
30 + yr old TB out 24.7 in winter was fed 1.5 scp nuts, 1 scp sugar beet, 1 scp Baileys No1 plus a cup of corn oil twice a day at my old yard. She did pretty well on it. We used to swear by Baileys No1 for keep condition on without hotting up. Barley rings are also good.
1 scoop Baileys Top Line Conditioning Cubes
0.5 Scoop of Mollichaff Showshine
0.5 Alfa A Oil
scoop Garlic
Recommended amount of Top Spec Balancer for weight
I know its a bit all over the place, but he is a fussy feeder and wont eat Speedibeet (doesnt even eat polos!!). Dont add oil cos all above is oil based and he would get diorehha.
I fed him on Spillers Senior Conditioning Cubes, HiFi and Sugar Beat twice a day. He looked fantastic on it. I also added oil, and ensured that he had adlib haylage when in.
Baileys No. 4 (Top Line Conditioning Cubes) are excellent. They've put weight on my slim horse and kept it on.
If it were mine, Id make sure it was rugged up the eyeballs and have a worm count and teeth done. If ok, then Id be feeding the No.4 at the highest recommended level along with something like Alfa A Oil and Alfa-Beet. Id also be adding a high spec allround vit & min supplement (high spec being it has B vits which some dont). Otherwise Id be feeding as much hay / haylage as she wants. Id also work out how she'd like to be kept (if you have the choice) as some horses get stressed being stabled all the time for example so turning out might make her happier and thus allow her to eat more and digest it better.
All three of my horses aren't particulary good doers but all seen to keep there weight on with the following feeds - ex racer has scoop of spillers conditioning cubes, scoop of spillers conditioning fibre and half a scoop of dry weight speedi beet (in middle of winter change this to alfa beet or fibre beet), 20 year old tb cross has a scoop of spillers senior conditioning mix and scoop of spillers conditioning fibre and my 18 year old mare has scoop of saracen enduro 100 and scoop of conditioning fibre twice a day. They all have hay,readigrass and a snack ball with hi-fibre cubes in when they are stabled. The enduro 100 is really good if you need to put weight on whilst still working your horse(mines in a riding school and does about 12 hours of lessons a week so no chance of a holiday to get the weight on!!) Spillers have a really good feed line to ring with any questions and so do dengie good luck