A friend of mine feeds Baileys No 4 to her draft horse and has had great results. She tells me that draft horses don't do well on hard feed, so are these 100% fibre? If so, what's the difference between these cubes and ordinary pony nuts?
Nope, they are a cereal feed, like pony nuts but based on wheat with soya oil added plus lots of vits and minerals and high quality protein to help with building muscle, the difference from pony nuts is the higher protein and oil content.
Also what about Lucienuts and grass nuts?
These are a fantastic option, I would always try grass nuts before a conditioning 'hard feed' because at this time of year they can just be lacking 'wet forage' which these can replace
She came from sales to the dealer to a few days later me! She was only on hay as far as I know.
Dont get me wrong she has put weight on, but its seing those hip bones that bug me, she now has a covering on her ribs (untill she turns her head and they poke through!)
Given you have her on an already nutrient rich diet only 1 month after just getting hay and that she is under weight (though already improving) I would be very careful about adding much more just yet. If she was moderatly thin before (ribs visable etc) and she is now starting to cover up I would stick with what you are doing, increasing her weight too quickly could upset her tummy and the last thing you want is colic or something similar to deal with

I would expect it to take around 3 or more months to get a horse from moderatly thin back up to a reasonable weight

some take even longer than that.
And don't worry too much about those hips just yet, my anglo arab is pleanty fat enough (has been on a diet all winter) in that you have to feel for his ribs and he still has some fat behind his shoulder but his hips still poke out a little bit

its just the way some horses are until you get them muscled up
