welcome to topvet

Feeding a horse is one of the most important responsibilities of a horse owner. If not done correctly and overfeeding grain feeds a horse can develop colic, (build up of gas) it is caused as horses are unable to burp. There are 10 rules to feeding horses these are -
FEED LITTLE AND OFTEN - Horses have quite small stomachs so food must be fed small amounts at frequent intervals. Ideally a horse should never go more than 8 hours without feed.
PLENTY OF BULK - To work efficiently a horses digestive system needs to be constantly filled. To achieve this feed bulk feeds such as hay and grass.
CORRECT AMOUNT AND TYPE - You can cause your horse physical and mental problems feeding too much, too little or the wrong type of feed.
MAKE NO SUDDEN CHANGES TO DIET - Introduce new feeds gradually to avoid digestive problems.
ROUTINE - Horses are creatures of habit.
WATER BEFORE FEEDING - A horse can wash undigested food out of its stomach if he drinks after feeding.
Also when feeding horses there are other considerations these include -
THE WEIGHT OF THE HORSE

THE BREED

TEMPERAMENT

AMOUNT AND TYPE OF WORK

TYPE OF RIDER

WETHER FULLY STABLED OR AT GRASS

TIME OF YEAR

TYPE OF PASTURE

AGE OF HORSE
17 hands - 34lbs

16 hands - 30lbs

15 hands - 26lbs For the horses and ponies who are half a hand or 2 inches

14 hands - 22lbs higher, an extra 2lbs is added example 13.2 hands - 20lbs,

13 hands - 18lbs

12 hands - 14lbs
A horse in light work should get 30% concentrate 70% bulk.

A horse in medium work should get 50% concentrate 50% bulk

And a horse in hard work should get 70% concentrate 30% bulk.
Most feed merchants have balanced diets for all type of horses and all types of work. Most feed merchants have dieticians so you can ask them or your vet what to feed your horse. Always insure your horse has access to clean fresh water. A horse can drink between 10-15 gallons a day.
In other way how to feed your horse vitamins properly
Consider the following vitamins that your horse may need. Many of these vitamins (like those in human diets) are provided in their everyday foods, i.e. the standard feed/hay that your horse is receiving. What sort of supplements your horse needs is determined by its age, activity level, whether you want to breed him/her, your horse's condition, feed given (pellet, 10/10, oats, first/second cutting hay), etc. Your veterinarian will help you select what to add to your horse's feed (be it squirts of vegetable oil, ground orange peel, glucosamine, bute...)
o Vitamin A (found in Carrots, Colostrum Milk, Fresh Grass and Herbage)
o Vitamin B1 (Alfalfa)
o Vitamin B12 (Cod Liver Oil, Soy Bean Oil and Fresh Grass and Herbage)
o Vitamin B3 (Peas and Beans)
o Vitamin C (Carrots, Apples, Fresh Grass and Herbage)
o Vitamin D (Colostrum Milk, Fresh Herbage, and Sunlight)
o Vitamin E (Alfalfa, Cereal Germ, and Fresh Herbage)
o Vitamin K (Alfalfa, Fresh Grass and Herbage)
o Biotin (Fresh Grass and Herbage, Maize, Rose Hips, Yeast)
o Calcium (Alfalfa,Limestone Flour, Seaweed, Sugar Beet)
o Phosphorus (Bran, Dandelions Oats, Barley, and Maize)
o Protein (Alfalfa, Cereals, Fresh Grass and Herbage, and Linseed)
o Manganese and Zinc (Alfalfa, Bran, Cereals, Fresh Grass and Herbage, and Yeast)
If your horse is in a stall during feeding, this is relatively easy. If your horse isn't picky, you can just take his feed bucket, add pellets (or whatever he gets) and sprinkle the vitamins on top (give a spritz of oil, whatever,) then give him the bucket. If your horse is being rough boarded (i.e. It is difficult (to put it mildly) to feed only one horse in a pasture if they're all hungry, unless your horse is the dominant one. See about what the regular feeding time is for the other horses in that pasture, and feed him then
Generally, it's best to have in a large carry bucket enough of the feed you're using to feed all of the horses in a given pasture at the start of feeding time. Which ever horse is waiting at the first bucket, put about half of the food he'd normally eat at meals in the bucket. Sometimes they'll vie for buckets, claim someone else's, etc.
After a minute or so, each horse will start to settle on its bucket. Head back to the first horse in the line's bucket, and fill your scoop with the remaining half of his food, and on top of that, sprinkle his particular vitamins for the day. Dump in bucket, move to next. Be aware, this has to be done relatively quickly, as horses tend to finish their feed fast. To avoid this, sometimes a zig-zag feeding approach helps - feed the first horse, then the next, notice that the first has really settled on his bucket, go back and add the second half and vitamins, go feed the third, notice the second is settled, etc.

Warnings
Don't choose your vitamins or supplements based on what your friend is giving her horse. While that Wheat Germ Oil, or "magic supplements" to stop your horse from chewing wood may sound great when you're friend is describing it, you definitely want to talk to your vet before adding anything new to your horse's diet. Each horse is different, and even very similar horses may have different dietary needs based on minute differences in build, condition, and temperament. Even though it's easy to order vitamins online, unless you're fairly experienced, you shouldn't assume it's safe to just add vitamins as you see fit to your horse's diet