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I feed Polly Hifi Lite as a direct hay replacer, weight for weight as part of her feed. Other than that she gets Feed Balancer (Enhance), Herbichop and Sugarbeet (Ultra beet), garlic and Steady-Up, and carrots. She only has a token feed when the others in the barn have their feeds. The feed balancer ensures she gets all she needs, stuff that her hay might be lacking. Smartie just gets feed balancer and chopped carrots in a decahedron toy. Other than that at least 90% of Polly's intake is hay and 99% of Smarties intake is hay. They are both on maintenance based diets.
The best people to ask are the free advice helplines that are provided. I phoned them when I was working out the feeds for my horses and found them very helpful. Also you could try looking on the back of the feed sacks as it should give you a guide for feeding on there. You should make sure you know how much your horse weighs before doing all this as feed rations are based on roughly 2% of a horse's body weight.
You've got nothing to lose by asking them and they will know a lot more about nutrition than the majority of people on here, including myself, it's what they get paid for!!
Jo