I fed my rescue mare a mix of weight lifter, copra, and millrun twice a day with 2nd grade lucerne within 2 months she had gone from a body score of 1 to at least a 3.5. Weight lifter is $$$$ but well worth it. I have now learnt to make my own.
1/2 - 1 cup sunflower seeds
2 dippers Mix of chaffs (lucerne, barley, oaten)
1 dipper millrun or rice pollard
1 dipper copra
I dont use pellets on mine, attempted the calm performer....horse not so calm

I have also used Bio Mare with some success too

I also added apples, carrots, whole cooked and mashed pumpkin to their mixes too and they are fat now. I agree that Speedibeet is great and economical too. Now that mine are in good cond. they get Speedibeet, millrun and copra in the morn with a biscuit of 2nd grade lucerne and a big biscuit at night. They keep their weight on well

Hope this helps