I like to define the equine diet by three main elements - forage, energy and other nutrients. We need to understand these elements separately before combining them as components to build a total ration.

Like other herbivores, horses' digestive systems are designed to continually process small amounts of feed rather than two or three meals daily.

Trying to use a single product or feed to optimize all three elements often results in excess calories and energy levels, or in providing minerals at less than minimum requirements.  Understanding the main elements of the equine diet will help you choose the best combination of feeds for your horse. 

The foundation is forage -
Forage usually forms the major portion of the equine diet and can provide most of the protein, energy, major minerals and vitamins needed for the majority of horses while furnishing the fiber essential for maintaining good gut health.

Forage comes in many forms - pasture, rangeland grasses, cured hay, hay products such as pellets, cubes, haylage, chaff or "chop", and browse.  In most places, when we think of horses we think of pastures in the summer and hay in the winter. In some parts of the world, forage is defined by what is available and may include crop stalks, copra (from coconuts) and other native fiber sources. Browse - leaves, seeds and twigs from trees and bushes provides variety.

Because horses produce stomach acid all the time (not in response to food like us) it's important that they have a source of forage available as much of the time as possible to prevent ulcers.  

Energy comes in many forms -
Some horses require more energy (calories) than they can get from their forage. Growth, training, hard work and making babies all require more calories than sunning in the pasture or light work.  

The primary energy source for horses comes from the fermentation of fiber (complex carbohydrates) from forage in the hind gut. The cecum and colon function like a huge vat, where billions of friendly bacteria continually ferment food, changing it into volatile fatty acids (VFA's)  which can be transferred across the wall of the large intestine into the blood stream.  Some VFA's can be converted to fuel to be directly utilized by the cells while others are converted to fat or glucose for future use.

Simple carbohydrates are digested by enzymes and absorbed in the small intestine and can provide quickly available energy.  Simple carbs can also be fermented in the hindgut. However, a large amount of simple carbohydrates that are spilled over into the hindgut - say a large grain meal that is incompletely digested and absorbed in the small intestine - can overwhelm the bacteria in the hindgut that do the work of fermentation. 

Fats can be used as  an energy source, but it is a two step process which requires oxygen and utilizes glycogen for conversion to fuel.  It takes time for the body to adapt to using fat as fuel and is not as quickly available as glucose when a burst of energy is needed. Large amounts of fat in the feed can inhibit absorption of other nutrients and, in susceptible horses can precipitate insulin resistance.

While important for providing amino acid building blocks protein is a poor source of energy, utilized only when other sources are unavailable.  The complex conversion of protein to energy is costly and can result in muscle wasting, liver and kidney problems.

Vitamins, Minerals and other Nutrients
Some minerals may be deficient in forages while others, notably iron, are present in excess and some minerals share absorption pathways. Both excesses and competing pathways can result in a "relative" deficiency which can be avoided by balancing to hay analysis results.
Major mineral balance is especially important for young stock and horses in training and hard work. Calcium to phosphorus and calcium to magnesium ratios can affect bone mineralization and muscle recovery. Simply adding major minerals without first knowing the actual levels in your forage can result in skewing the ratios rather than improving them.
Horses with access to pasture and other "live" forage have little need for supplemental vitamins while horses restricted to hay or other cured forages can benefit from vitamin E and Omega-3 supplementation.  Vitamins A and D remain adequate in cured hay for some time; over supplementation of these is more likely than deficiency.

When protein requirements exceed what's provided by forage and grain, protein levels can be improved by adding concentrated protein sources such as whey protein isolate, split peas or pea meal (more common in Europe) or legume (alfalfa) hay or pellets. Smaller amounts of highly digestible complete proteins can replace larger amounts of "crude protein" (CP) - replacement does not need to be done on a gram by gram basis.