As you know, if you follow the Feed Rules, the number one rule is that the majority of your horse’s diet has to be made up of plant material (forage).
A horse in a crowded pasture on the Mongolian steppe
Unless your horse lives out in a pasture (or the Mongolian steppe), chances are that your horse doesn’t have constant access to growing plants. Thus, you’re most likely going to have to provide your horse with some sort of dried plant product: most commonly, that would be hay.
For some reason, people seem to get very emotional about hay. I don’t really get it – there are records of hay being fed to horses from about 500 BCE, so you’d think if it was all that awful, someone might have figured it out by now. But, in fact, hay (which comes from an old German word, hewi, referring to “that which is mowed”) has pretty much allowed for horses to be domesticated.
Alfalfa hay
Depending on where you live, you’re going to have access to any one of a number of hays, from alfalfa to timothy, from coastal bermuda to lucerne, if plants can be dried, they can usually be fed to horses. Of course, there’s some variability in hay, depending on the time that it was cut, conditions under which it was dried, but they all have a lot of things in common: protein, fiber, energy, vitamins and minerals, in varying amounts.
Even though there are lots of different hays, they can all be roughly divided into two types. Legumes are plants that have microorganisms associated
Timothy hay
with their root systems; alfalfa hay is the classic legume. Hays made from grasses and cereal grains (such as barley or oat) make up the second group. You can make a great diet for horses using either type of hay as the foundation, but there are some differences. In general, compared to legume hays, grass and cereal grain hays have:
1. Less protein (but plenty of protein for adult horses)
2. Less calories (which means you might have to feed more, and which makes them great for horses that need to diet, because you can feed fewer calories and still fill up your horse’s stomach)
3. Less calcium (but plenty of calcium for adult horses)
4. Fewer vitamins (but plenty of vitamins for adult horses)
5. More digestible fiber (but legumes have plenty of fiber, too)
The higher protein, calcium, and calorie content of legume hays make them excellent for feeding to young horses; the extra protein, calcium and 
calories pose no danger to older horses, but you might find that if you feed your adult horse too much alfalfa, and you don’t give him enough exercise he’ll get fat (by the way, in case you didn’t know, this can be a problem in people, too).
How much hay should you feed? A rough guideline for an adult horse that’s not in heavy work is 1.5% – 2% of his body weight. That means for a horse that weighs 1000 pounds (say, your average Quarterhorse), you’d be looking at feeding 15 – 20 pounds of hay per day. And here’s a money-saving tip; if you get a feed scale, and weigh the hay (as opposed to measuring it in, say, “flakes”), you’ll save money, because you won’t waste hay.
You can do a lot of things with the plants that make hay after you dry them. People will chop them up, and mix them with molasses (for example, “A&M” is alfalfa and molasses), steam press them into little cubes, or dice them finely and press them into pellets. All of these products can provide good nutrition for horses, and none of them are as bad as people try to make them out to be.
The bottom line, when it comes to feeding hay? Feed a good quality hay (no dust, no mold, lots of green color). Feed it in proper amounts, and don’t let him get fat (make sure you can feel his ribs). If you do that, you’ll go a long way towards making sure that your horse is as healthy as, well, as healthy as a horse!
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