Forage (grass and/or hay) should be the basic building block of any equine feeding program. Horses are designed to eat fibrous plant material, preferably almost continuously throughout the day for optimal health. Humans have altered that healthy pattern of eating by domesticating horses, housing them, and giving them jobs to do.
All horses should have a minimum of 1% of their body weight in long stem fiber each day. For the average 200 lb adult mini, that is 2 lbs of grass (by dry weight) and/or hay each day. On average, 1.5%-2% of body weight in hay or grass will keep an adult maintenance horse in adequate weight. Harder keepers or moderately worked horses may require 3% of body weight, and that's about as much bulk as any equine's system can handle in a day.
The nutritive value of hay, however, is totally dependent on the type of plant (grass vs. legume hay such as alfalfa or clover), the geographic area where it was grown (plants uptake minerals and nutrients from the soil and soil types vary across North America) plus the conditions under which it grew and was cut (rainy, drought, cool, excessively hot conditions, rained on after it was cut, etc., etc.) It is just not possible to know the amount of nutrition any crop of hay will provide to your horse just by looking at it; to know for sure it must be analyzed at a Lab. For starters, however, you can determine the type of hay and research the nutrition tables of various types of hays for average figures for nutrients -- always keeping in mind that the figures in the table will only give you a rough idea of what your hay may be providing to your horse in terms of protein and micronutrients. Indeed the hay-only diet will undoubtedly result in gaps in the overall nutrition profile. A hay analysis will tell you where those gaps are and the most meticulous of horse owners (i.e. owners who are dealing with Cushing's, EPSM, I/R, chronic founder, etc.) may have custom mineral mixes made to match their hay, thus ensuring a more completely balanced diet. Most of us, however, don't have the luxury of purchasing enough hay at one time to make an analysis useful, so we are left with a little guesswork. Since the hay-only diet undoubtedly has SOME areas where it is nutritionally deficient, the most prudent thing to do would be to offer a high quality vitamin/mineral supplement alongside -- not one that they can eat free choice, but rather one that they eat just like a meal and is given in the amount recommended by the manufacturer. This might be in the form of a no-grain ration balancer, or a product like Horseguard, Accel, VitaKey, Select I or II, etc., etc. The choices of vitamin/mineral supplements are mind boggling when one looks at all the offerings via catalogue or in the stores.
However, to answer your question the bottom line is that for the average, adult maintenance horse in good health and good weight, a hay only diet PLUS a vitamin/mineral supplement, salt, and good clean water can be an excellent dietary program.
One downside to the hay-only hay diet is the possibility that your horse may develop a bit of a belly. This is not a bad thing -- just reflective of the fact hay takes longer to digest than grain. A belly full of hay sits around in the fermentation vat called the cecum where it produces gas, sometimes creating a bloated look. This may become more pronounced after time as the abdominal muscles may stretch a bit, thus even your geldings may take on a "broodmare"-type look. Again, this is not injurious to their health, but many folks find it unpleasing to look at. I'm not sure if it's all that important for our pasture pets to look like show horses, and in fact mine do not. But this simple fact of distended bellies is one of the main arguments for NOT feeding a diet 100% comprised of hay. Remember that a horse REQUIRES only 1% of body weight in long stem fiber each day, and that could be any combination of hay, grass, beet pulp, alfalfa cubes, chopped hay, etc. That keeps his gut healthy, but it may NOT be enough to provide for his energy needs and/or to keep a horse in "show weight" and condition. To make up for what the hay lacks, we can provide grains for energy and weight gain. This could apply to broodmares, stallions, and young horses, too. Show horses, youngsters and broodstock generally have higher caloric and energy requirements than maintenance pasture pets do not have. Grains or commercial feeds containing grain can provide calories for weight and carbohydrates for energy. However, if your horse does NOT need extra weight and does NOT need extra energy, chances are you can readily feed an hay-only diet -- BUT don't forget the salt, plus the vitamins and minerals!!!
Robin C