Horses and other equines evolved as grazers, mainly eating plains grasses that required constant movement to obtain sufficient intake. They were exposed to geographic variety and also to seasonal fluctuations that not surprisingly coincided with seasonal reproductive status - late summer into autumn abundance for building up fat stores, grasses high in vitamins the mares need for the final in utero development of their foals, nutrient rich grasses through the summer to make milk without depleting the mare's own body stores of minerals.
Horses have adapted to the confined spaces they're often kept in; most of us grow up believing that horses belong in stalls eating hay and grain except when they're let out to eat grass or to be ridden. Few of us get to see horses in a truly "natural" environment eating a "natural" diet but we are becoming more aware of the benefits of trying to emulate a more organic lifestyle.
The foundation ration for all horses is based on forage – hay, grass, hay pellets or forage cubes.The nutrition value of these forages is not all equal – protein and energy levels, minerals and other nutrients vary depending on species, growing area, weather and depth of irrigation, fertilizing, and length of time since the forage was cut and cured. Hi yield fields may have depleted soil, and many growers still equate quality forage with the high protein and calcium levels favored for meat and milk production. It's often difficult to provide variety, so many horses receive the same basic fare year round.
We know the minimum requirements for many nutrients based on the horse's age, work level, reproductive status and health. These requirements are set forth as guidelines in the National Research Council's Nutrient Requirements of Horses - commonly referred to as "NRC". While these guidelines are based on averages and some information has been extrapolated across different breeds (and from other species) the NRC Requirements represents the best available information. Similar to the "daily requirements" established for humans, the NRC guidelines for many nutrients are minimums needed to prevent clinical disease rather than optimal levels that support peak health and performance.
Forages can provide most of the requirements for idle horses or horses in light work but some nutrients may be deficient or present in excessive levels. Hay and feed analysis is how we determine which nutrients need to be supplemented and helps to avoid over-supplementation; it will also identify excessive levels of minerals which, because of competing pathways, can cause "relative" deficiencies of others. Mineral balancing can effectively correct these deficiencies and excesses.
Pregnant or lactating mares, horses in heavy or intense work and foals, weanlings and growing horses have specific requirements beyond maintenance levels, while horses with metabolic issues such as Insulin Resistance may not tolerate feeds that traditionally benefit other horses. Yet all of these horses will begin with a forage-based foundation individualized to their additional needs. Almost all horses can utilize a low sugar/low starch forage as long as protein and energy levels are adequate. Except for some types of work, most energy calories the horse needs come from the fermentation of forage in the hind gut. Feeding the work targets the higher energy needs (and ability to tolerate carbohydrates) of the working horse.
The overviews here are not meant as complete feeding guidelines but as a way to become familiar with the basic principles of your horses' nutrition requirements. Whether you want to make better decisions when purchasing feed or want to evaluate your program for optimum performance, you'll find the books, classes and other information in the Resources section helpful. Dr. Eleanor Kellon's NRC Plus course, part of her total online equine nutrition program, can provide the beginning foundation for building your knowledge base.