The NRC is the National Research Council. They produce a book will all the requirements of horses (and other animals as well). They update the book quite often and go into a lot of detail. I will write another post on the actual nutrient requirements of the horse, they can be seen on this website however and vary from horse to horse.
The link below is to a website where you put in the specification of your horse and it tells you their nutrient requirements.
This can't be done accurately in practise. There are a number of problems such as you do not know how much grass your horse is eating, most people also do not know the nutrient content of the grass their horse is eating. This is probably done more accurately on racing yards where nutrition is very important and they will have their grass analysed. The nutrients of forage (grass, hay, haylage etc.) varies greatly depending on where it was grown, the time of year etc.
To use the website you put in the weight of your horse. The intake level is usually 2% of the body weight a day. You can then chose if it is an adult at maintenance, pregnant, lactating etc. Most horses will be on a low level of work, medium is considered to be competing horses and high would be for race horses.
You can then go on the dietary supply section and add things such as 5 kg of hay and see what the nutrients of this would be compared to what they would need (remember this isn't accurate). If you also know the contents of your feeds you can input these values into the tables.
Below is an example of a ration I made during a piece of coursework. It is important that the ration is largely made up of forage. Safe and Sound was a forage feed we looked at similar to Alfalfa, we used this as we did not have grass or hay to analyse. As you can see the figures do not match up exactly. This is very hard to do as there are small measurements involved. As with people, your horse may eat slightly more one day than the next, as long as it all balances out it will be fine!