All right, you didn't say so directly, but you seem to be in Australia. The feeds available in Australia may well differ from those in the UK, the US, and other places. Furthermore, even QL and Vic have substantially different climates, and thus may well have different diets that the horses will require and have available. WA is of course far away from just about everything.
Regardless, most horses can live adequately on just good hay and pasture unless they are doing strenuous work. Also, most horses carry a burden of worms which can be kept under control by proper medication and proper management of pastures. Your local veterinarian will have appropriate information for you. It does not make sense to pump huge amounts of feed into your horse just to feed the worms!
Our own horses are mostly old ponies, tough little critters. The common knowledge is that ponies live on air, and will founder on fat air. One of them is so old that his teeth are wearing out, and although he can get through the summer on grass he requires supplementary grain and pelleted grass in the winter.
Learn how to score your horse's condition. A good hands-on method is the Henneke score, developed by Don Henneke at Texas A&M.
Without more information on where you live and what you plan to do with your horse or horses, I can't really give specifics. I can only say that each horse needs to be fed individually, according to his/her age, health, and level of work. Horses with special needs, such as pregnant and lactating mares, breeding stallions, horses in heavy training or competition, and young, growing foals and yearlings, obviously require more feed than horses which are retired or doing only very light work. Horses also need to be fed according to their state of health, because they can suffer from some of the same diet related ailments which plague PEOPLE. Horses can suffer from insulin resistance and from nutritional deficiencies, just as people do. They also can develop ulcers. And two of the most dangerous illnesses horses get, colic and laminitis, are both directly related to diet and feeding. The number one cause of both these diseases is sudden changes in diet, in fact. In the case of laminitis, it's usually carbohydrate overload that's the cause- meaning that a horse has gotten loose somehow and has overdosed on sweet feed or other grains/feeds. Horses can and WILL eat themselves to DEATH if they're given the chance to do so, which is why it's critically important that items like grain be stored securely in a horse proof, locked location. With colic, the cause is often a sudden change from one feed to another. ALL feed changes must be made GRADUALLY when feeding horses. If you want to change a horse's feed, you need to start slowly and add in small amounts of the new feed over a period of a week or ten days, NOT ALL AT ONCE.
Other rules to remember when it comes to horse feeding are:
-Never feed a horse which is hot from work or exercise, as this can cause colic. Allow the horse to cool off slowly and then offer the feed once the animal settles down
-Grain and feed must be stored in a cool, dry place. NEVER, EVER, feed grain which has white mold on it, is excessively dusty, or which has been contaminated with mouse or rat droppings. YOU CAN KILL HORSES IF YOU DO THIS, so take a hint and AVOID this mistake.
-Allow horses to feed in a natural position, out of troughs with large bottoms. Said troughs or buckets should be placed at normal head height or lower.
-Make sure horses are free from DENTAL problems. Horses that have dental issues like sharp points, abscessed teeth, mouth sores, or infections won't be able to chew correctly, and will be at risk for both colic and laminitis.
-If you feed horses in a herd, make sure you keep an eye on them and that ALL the members of the herd get the appropriate amount of feed. Most horses which live in herds will establish a well defined pecking order. What you need to make sure of is that the horses at the bottom of the order get enough to eat.
- Provide plenty of FRESH, CLEAN WATER at ALL TIMES. This is another critically important part of good horse management, because horses which don't or can't drink enough fluids are in danger of a fatal colic. They must have fluids in order to digest their feed.
I have a 7yr Appendix mare I feed her 1/2 can of grain 1 flake of Alfalfa and 1 flake of Grass twice a day.
Every horse has different feed requirements. Some do fine on just hay and little or no grain. Some need special feed, others don't. You'll ask the person you buy the horse from what it's fed and continue on with that unless there is a need for something different. Many people feed way too much grain.
Although I'm all about good nutrition, I tend to follow my show trainer's philosophy. He has always fed a very basic, inexpensive sweet feed--and his horses look (and perform) like a million bucks. So unless the horse has special needs of some sort, as long as they're looking and performing well, I'm not changing the plan. I moved my older show horse home a few months ago and my husband and I own 8 total.
1. 15 y/o AQHA show horse, light work, stalled/turnout - a 14% senior pellet twice a day (6 qts. per feeding), plenty of hay when stalled and grass when turned out (he just moved up to my mom's barn because she's about to start showing him, and honestly I don't know the specifics of what he gets anymore). Easy keeper, large frame.
2. 4 y/o AQHA show horse-to-be, heavy work (1-2 rides daily, in training), stalled majority of time - 10% sweet feed twice a day (6 qts per feeding), 2 flakes of hay twice a day, a scoop (approx. 3 qts) of alfalfa (either cubes or compressed hay) twice a day, minimal grass during daily turnout. I've only had her home a few days but that's what she'd been getting and she holds her weight well so we haven't changed anything yet. Light frame (still immature).
3. Yearling AQHA stud colt, on pasture with minimal grass, light work (short lunging or groundwork sessions 2-3x per week) - 1 1/4 scoops (about 4 quarts) of 10% sweet feed 2x a day, 1 flake of hay morning and night.
4. 2 year old APHA mare, currently stalled, has been on light work but increasing - 2 scoops (6 qts) 10% sweet feed 2x a day, 1 flake hay morning and night, half a scoop (1.5 qts) compressed hay 2x a day.
5/6. Grade Paint gelding and grade QH mare, both 5-6 years old, on full pasture with excellent grass, very light work - half a scoop of 10% sweet feed twice a day. Easy keepers.
7. Mini, lives on pasture with minimal grass (keeps the colt company), no work - about 1/4 scoop 10% sweet feed 2x a day, small flake of hay morning and night.
8. 6 y/o AQHA gelding, ex-halter horse, lives outside with minimal grass, hard keeper, light/no work- 2 1/2 scoops (about 7.5 qts) 2x a day of Nutrena ProForce, 2 flakes of hay morning and night, 1 scoop of compressed alfalfa 2x a day.
Basically, the horses on the cheap feed look like a million bucks with shiny coats, and the one horse that gets the really good stuff looks pretty pitiful most of the time. But we do what we have to do ;).
Okay. Here's what I do for my horses in light, moderate, to intense work. I do nothing with one horse (she's not technically mine...) all the way to a thousand miles of endurance riding in a season with one in intense work. I give even the horse in no work a fair bit of supplement because it is what she needs to maintain weight properly. She does run a lot on her own because that's how she is. You feed each horse based on that particular horse....
Hay - about a half bale per horse per day (about 20 pounds). Pasture 2-3 hours a day usually in summer (I live in a winter climate).
Grain and grain like supplement. 2-5 pounds a day. I use a homade grain mix of soybean, oats, and corn with mineral, salt and oil added. They get a little of this each day (1-2 pounds). And I have them on a high fat pellet that has probiotics, prebiotics, yeast, flax, rice bran, balanced vitamins etc.... etc... etc.... (1-3 pounds). I use Nutrena Empower Boost. Need the fat and I know it likely contains things they don't necessarily need but they work hard and I don't want them shorted.
Beet pulp - 1 to 2 quarts of dry measure with equal volume of water added
Flax seed ground - 1 to 2 cup a day (source of omega 3 oils)
Brewers yeast - 1/2 cup a day (prebiotic source)
Aloe vera juice 1/2 cup a day (calms stomach)
Thia-cal B vitamin and Calcium supplement (for nerve function and buffers stomach/good cardiac function) 2 oz a day.
Omeprazole recommended preventative dose per day for horses in competition or anyone trailering. Sucralfate after trailering/stress (for some stupid reason that makes no sense to me it is illegal for my competitions...). These drugs prevent and treat ulcers and most horses do have ulcers. Competition horses nearly all get them if untreated.
And they each get about a pound of carrots each week and apples in apple season. Lots more carrots when I'm in competition because they are a low glycemic index food I can use in place of grain in the morning before the stress of an endurance run. My horses all run a body condition score of 4.5 to 5.5 depending on the horse and time of year. You want to shoot for 5. One tends toward fat (haflinger pony) and one toward thin (thoroughbred) based on their body metabolism they were born with. And one horse is really average metabolically (Arabian). I feed them three times a day. In summer, I push my thoroughbred to eat 24/7 whenever he can and he drops a little weight from all the running but hey- he loves to run and routinely tries to gallop off with me just because it's so much fun. He eats SO SLOW...... The pony I have to watch carefully his portions and he's a little hoover. In winter he gets just a small amount of grain or he would be the size of a small tank with legs. Eats really really fast. And the Arab is very average.
I'm a bit on the high-end side of food complexity and cost because of what I ask my horses to do. Good nutrition is important for everyone but especially the performance horse. Many people feed just a good clean hay and very little for a ration balancer/supplement.