Horse nutrition has become overly complicated over the years - but covering the basics will take you a long way.
Hay should be made available to stabled horses at all times (except just prior to racing), lucerne hay is palatable and helps to reduce risk of ulcers, however sometimes a mix of hays is required (e.g. lucerne/clover or lucerne/oaten) to prevent unnecessary weight gain from lucerne alone. Slow hay feeders/hay nets are useful for slowing intake and reducing wastage.
Grain is required to fulfil energy requirements of heavily working horses. The amount and type of grain used will vary from stable to stable depending on how horses are worked and environmental factors. Oats are highly digestible and a good choice, steam flaked or micronised barley is useful for horses that require a "cooler" or more "conditioning" diet, corn is energy dense and is useful as an energy supplement. High quality protein sources are required to meet amino acid requirements. There are benefits to adding fat to the diet of racehorses, vegetable oil (e.g. canola oil) can be used but should be high in quality (cold-pressed) and fresh. It takes time for horses to adapt to using fat as an energy source so it should be introduced early in the training program (during pre-training). Last but not least the diet should be balanced, with more than 25 minerals, trace minerals and vitamins needed for optimal health and performance.
Mixing a small amount of chaff through the feed mix will help to slow intake and improve nutrient digestibility. Splitting the feed into at least two meals daily will help to distribute intake of protein/carbohydrate and other essential nutrients throughout the day (i.e. before and after work).
The stable mix should be able to suit the needs of the majority of the horses allowing for increases/decreases in feed offered for individuals to optimise bodyweight. More tailored feeding programs (e.g. for tying up) can also be created by modifying the energy source used.
The end result = a stable feeding program that provides nutrients for fuel, recovery and health and ultimately optimal performance.