The first thing we need to remember about horses is that they aren't modern. The horse has developed over several thousand years to eat grass. So how does it cope with the modern roughage, hard feeds and feeding regimes?

-Mouth

Horses select pieces of forage and pick up finer foods, such as grain, with their sensitive, prehensile lips.

-Esophagus

The esophagus is about 1.2 to 1.5 m (4 to 5 feet in length, and carries food to the stomach. A muscular ring, called the cardiac sphincter, connects he stomach to the esophagus.

-Stomach

Horses have a relatively small stomach for their size, and this limits the amount of feed a horse can take in at one time. In the stomach, assorted acids and the enzyme pepsin break down food. Pepsin allows for the further breakdown of proteins into amino acid chains. Other enzymes include resin and lipase.

-Small intestine

The horse’s small intestine is 15 to 21 m (50 to 70 feet) long and holds 38 to 45 L (10 to 12 US gallons). This is the major digestive organ, and where most nutrients are absorbed. It has three parts, the duodenum, jejunum and ileum. Horses do not have a gall bladder, so bile flows constantly. Most food is digested and absorbed into the bloodstream from the small intestine, including proteins, simple carbohydrate, fats, and vitamines A, D, and E. Any remaining liquids and roughage move into the large intestine.

-Colon

The large colon, small colon, and rectum make up the remainder of the large intestine. The large colon is 3.0 to 3.7 m (10 to 12 feet) long and holds up to 76 L (20 US gallons) of semi-liquid matter. It is made up of the right ventral (lower) colon, the left ventral colon, the left dorsal (upper) colon, the right dorsal colon, and the transverse colon, in that order. Three flexures are also named; the sternal flexure, between right and left ventral colon; the pelvic flexure, between left ventral and left dorsal colon; the diaphragmatic flexure, between left dorsal and right dorsal colon. The main purpose of the large colon is to absorb carbohydrates, which were broken down from cellulose in the cecum. Due to its many twists and turns, it is a common place for a type of horse colic called an impaction. The small colon is 3.0 to 3.7 m (10 to 12 feet) in length and holds only 19 L (5 US gallons) of material. It is the area where the majority of water in the horse's diet is absorbed, and is the place where fecal balls are formed. The rectum is about 30 cm (1 foot) long, and acts as a holding chamber for waste matter, which is then expelled from the body via the anus.

So to simplify:

the horse chews and swallows

stomach is tiny (comparative to animal size) because they are used to eating small amounts constantly. here bits are broken down into smaller bits

in the small intestine proteins, vitamins, fats and simple carbs are absorbed

colon is where water is absorbed. also carbs are absorbed here, see why below.

I would like to add that the structural parts (cellulose) of a plant cannot be digested by mammals. The horse deals with this by keeping what's called a microflora in their hindgut, which consists of bacteria etc. These bacteria break down the fibres so that the nutrients can be absorbed. (Furthermore, in older plant material there is something called Lignin, which not even the microflora can break down. This is present in wood, and also in straw, which is why these have a low nutritious value for horses.) As all plants contain cellulose this is an important part of the horses digestion. This brings me to one of my main gripes with modern horsekeeping.

Haylage(!)

Haylage is practically pickled grass. It is softer than hay. It contains more nutrients than hay. It has a "nicer" taste than hay (ask any horse). It's obviously better than hay. Or is it? It has a different pH than hay. (As i mentioned before, equines had a long time to get used to hay.) Haylage is modern. It has less cellulose than hay. or grass for that matter, as the breakdown process of cellulose has already started before the horse eats it (the pickling effect). Therefore more nutrients become available earlier in the digestive system of the horse, in the foregut. Great, quicker nurtrients! Right? Or maybe it is not so great that the hindgut with its microflora is losing its job. Horses are not made for quick nutrients. They are made for roughage. I'm not saying haylage should be banned from stables, the same as I am not saying that hardfeed should be. But it should be considered just that, a hardfeed. Or a mixture of both. But definately not as a pure roughage. As straw has such high fibre and low nitritional value, they can both be mixed, or hay and haylage can be mixed. Although the horse will always (nearly) eat the haylage first, beacuse it is sweet. Many horses have a bad stomach after switching to haylage. One stable I have been to the horses had pretty much constant diahoarrea. They tried everything, all kinds of medication from the vets, feed supplements, over the course of several years. Some horses have gotten better, some not. I am still waiting to see what happens when/if they change to hay.

A lot of these problems stem from a lack of knowledge. People want a higher protein and energy food for their performance horses. Of course you wouldn't feed yourself on a diet of only pickled stuff, but surely when it is made for horses it must be good for them? I wish people took some time to research what they are feeding their animals instead of just believing what their trainer/local feed shop assistant/etc tell them. Of course it is more convenient with haylage, it can be stored outside (although it should not be kept in extreme weather conditions, not in direct sunlight or temperatures below freezing). Horses can be fed less hardfeed when they eat haylage (although most people don't), which is of course a bonus, horsekeeping is expensive. But the big con is still that horses are simply not made for eating it.

Conclusively I think that haylage can be fed in moderation as long as acidity and nutritional values are taken into consideration, but I do not think it should be a substitute for hay.

Hay and hardfeed

Hay, hay, wonderful hay. Is it really all that great though? It always contains some dust and/or mould spores. That can't be good, can it? And of course it isn't! But the horses are used to hay, which menas, they can digest it. You can get rich hay or poor hay, or anything in between. No two lots of hay are the same, If they've grown in the fields next to each other there is only a slight difference, but there is still a difference. I would always recommend an analysis, not only the basic one for energy and protein etc, but one that gives you all the mineral levels etc. The same i would recommend feeding straight grains. You can be more precise with this method. if you know what energy, proteins, minerals etc your hay contains, you can supplement with your hardfeed to achive the correct ratio of minerals, the correct level of energy for just your horse. But nowadays the ready mixed feeds win out. They are so convenient. And i must agree, they are! But sometimes they are filled with all-sorts.. It's like a pick-and-mix. I've been told before, by feed merchants and by the companies that produces the feeds, that the feeds vary depending on season. Not that they vary to help the horses, but that they vary depending on what is available to mix the feeds from, to keep costs down. Now, unfortunately i heard this from my favourite feed company, so I was suitably outraged, especially since they were talking about their own feed, not bad-mouthing someone else. I understand it, and they do make sure that the ingredients add up to the right level of energy and protein and fats and what not, but if they can vary from batch to batch that can't be great for the digestion either? If you feed straight grain at least you know what your horse is eating. And it is cheap. And hard work, especially if you have 20 or so horses and they all eat different stuff. On the other hand, you don't have to keep ten different types of musli/pellet feed for them, straight grains can be mixed and matched to suit any horse.

It is important to keep in mind that there are different "strengths" to hay, a little round pony should have poorer hay than a large gangly horse for example.

Straw

Contains lignin which can't be digested. I was told when I worked in a hunters yard that horses colic if you give them straw. This was the response to my suggestion of giving them just that to help the horses cope with the extreme amounts of haylage they were getting (half of it just went straight through them). I am yet to see a single horse colic because of it. I have seen a showjumper devour a whole stable full of straw over one night at a show, and still being taken out to jump in the morning (it knocked poles with its stomach, it was so big) and yet it did not colic. I don't think they should be fed only straw, and I don't believe excessive amounts are good either, but in moderation it helps give horses something to do (chew, which is what they all want to do, all of the time) without adding more nutrients to their diet. My horses have hay + feed and then they have a large pile of nice straw in case they feel the need to eat something else. This helps weight control as mine tend to get a bit too tubby otherwise. If they do more work I up the hay/feed and they themselves lower the straw intake.

Supplements is on a whole other level. I will not discuss them here. Honestly, I don't know much about the nutritional values in supplements, so I can't comment on it. One thing not many many people think about though, is that although the active ingredient may in itself be beneficial, the stuff it's mixed with could be less so. It could even be detrimental. I'd suggest adding with care. Does your horse really need it? And then, a few months later, ask yourself if you horse still needs it?