Fortunately, horses do quite well on a forage based diet of hay along with minerals and good clean water. However, some hays may be deficient in an essential nutrient or, depending on the breeding status and activity level, some horses may need more or less hay in order to be in good body condition.
The most respected research on horse nutrition is provided in the "Nutrient Requirements of Horses" (6th revised edition) by the National Research Council. This volume is a technical resource for academic purposes, but it does provide data and algorithms for calculating a given horses nutrient requirements.
This calculator implements the computer model based on this research.
Watch this short video to learn the basics of horse nutrition.