In the US, grass hay can have a nutrient level ranging from very low to quite high. 'Improved grasses' like Bermuda Grass can be quite high in protein a quite nutrient dense. It is affected a lot by fertilization and stage that it is cut. The earlier it is cut, the higher the protein content. Regardless of how it is grown and cut, most grasses are very low in Calcium.
Cereal grain hays are usually very high in sugar content and quite nutrient dense if cut early. They are also low in Ca and high in P.
Clover, Alfalfa and other legumes are much higher in Ca and, if cut and cured right, are also higher in protein. Some kinds of clover are very difficult to get cured well and can be dusty. Some clover can also make horses slobber and drool.
Generally, a person's feeding program must be built around the forage that is available in any given area and must be built around any metabolic issues a horse may have. Mature grass hay is really safe to feed mature horses and can usually be fed free choice. Nutrient dense hays must be fed in smaller portions because horses will over-eat it and get obese. Most grains are very high in P and very low in Ca. One can easily add protein to a horse's feed if they need it and one can add Ca, Mg and vitamins.. There is no way to take out any nutrients.