HOW MUCH TO FEED AND FEED CHARTS
I have had many varieties of horses and ponies in various workloads. I believe that the horse can be kept in a healthy state by having his teeth and back checked regularly, being on a good worming programme, a good routine and the vet involved, with bloods taken, if they do not look as good as they should.
I turn the horse out on grass as much as possible, with adlib hay in the winter and, if stabled, they have adlib hay or haylage all day.
I feed a handful of cool nuts when they first come in. This makes them want to come in, and stops them expecting feed at any other time.
If they are not holding their weight as well as they should and I have checked everything, I would use a conditioning mix and add vegetable oil (a good glug). I would only increase this so that the size of the feed never exceeds the size of a rugby ball for a 16h horse, smaller for a pony. I would feed 4 feeds during the day, with the largest being in the evening.
Weigh how much your scoop holds so you can check the amounts you are feeding.
My scoop holds 1.5 kg of cool nuts or mix.

I do think people make feeding so complicated when common sense and correct investigation should be enough. Below is the recommended way to feed, but I would not get too worried. If your horse looks good, you are following the rules of feeding and have looked into your feed well, then that is usually enough.
One fact of which I must remind you is that, if your horse’s intestine is empty for 4 hours or over, the healthy bacteria that breaks down the food starts to die. This is why it is really important that, even if your horse is overweight, he does not go for long periods without feed.
I have written this chart as a very rough guide for how much you should feed per day
Approx 2.5% of their body weight
to try to make it easy for you to remember. You should feed according to your horse’s weight, not height.
Following the recommendations given by the BHS set out below, would mean your horse has little hay for the night, so it may have an empty intestine for a good while. This could result in your horse suffering with ulcers, which is very common now. I believe it would be better to soak the hay, so it is almost all water and fibre, and feed more through the night so you keep a healthy gut.
There are many weight tapes that you can get to estimate his weight but the best way is to get them weighed, many shows will do this or it can be done at the vets.
If you do not have this luxury, then a very rough guide is- heart girth (measure his barrel just behind his elbow in inches) x heart girth x length of the horse (from point of shoulder to back of haunch) then divide this number by 660. This gives you his approximate weight in kgs.

To estimate how much to feed your horse, you take his weight then divide by 100 and multiply by 2.5.
a 500 kg horse will eat 12.5 kg
500/100 x 2.5 =12.5
This amount is the total feed weight, so this includes all bulk (grass, hay and haylage) and concentrates (hard feed like pony nuts, mixes and grain).
Now this is where it starts getting a little complicated so do one step at a time.
So if I had a 16h thoroughbred who weighed 450kg (a little thin) but working about three times a week for about 2 hours I would feed
450/100x2.5 =11.25kg in total
(using the equation from above). I will make it up to 13kg as I want him to gain weight.

He is in medium work so 40% concentrates (to work out percentages times the weight by the the percentage and divide by 100) so-
13×40/100=5.2 kg in concentrates
7.8 kg in bulk
13 kg in total
For his concentrates, I would feed a calm and conditioning mix. I would break the feed up into 3 meals so he has the best chance of absorbing it.
morning feed 1.5 kg (1 of my scoops)
dinner feed 1.5 kg
night feed 2.2 kg (1 and a half of my scoops) total 5.2kg.
I would also feed grass nuts (make sure they are soaked) half an hour after his feed and as much as he wanted.
I would feed a mix of hay, haylage and as much turn out as I could throughout the day to gain the weight he needs.

If I had a 14h cob who weighed 425kg (so overweight), in medium work, I would feed:
425/100x2.5 =10.6 kg in total
He is in medium work but overweight so I would only feed 30% concentrate.
10.6×30/100= 3.18 kg in concentrates
7.4 kg in bulk
For his concentrates I would feed a cool mix so-
morning feed 1kg
dinner 1kg
night 1.1kg total 3.1kg
I would feed soaked hay, which has less feed value, broken down into-
morning 1kg (weight before soaking)
dinner 2kg
evening 2 kg
night 3kg (soaked for at least 2 hours)
Final Thoughts
Again, I repeat, do not get too caught up in this.
Look at your horse, investigate feeds and use your common sense and try to keep as natural as you can.
The above recommendations are adapted from the BHS guidelines. In reality, as I have said before, I feed my horses who are in medium work, ad lib hay or haylage and a cup of cool nuts a day. They are turned out at night, so have grass as well and they all look great with lovely shiny coats and are very healthy in general.
If in any doubt, please consult your vet or a professional who has no vested interest in what you feed your horse.