Going back to the NRC tables we see our 1102 lb horse in light work requires 820 grams of crude protein (CP) a day. (A nutritionist will use a more detailed formula, but the tables provide a good starting point for most horse owners.)
Hay can range from a low of 4% protein up to 14% or higher. If you've had your hay analyzed or have been able toget a good idea of your regional averages, you can tell if your hay is meeting your horse's protein requirement.
For example, let's say your grass hay is 8% protein and your horse eats around 18 lbs of this hay each day.
8% / 100 x 453.6 = 36 grams of protein per pound. 36 x 18 = 648 grams of protein per day.
You can see this will leave your horse short 172 grams of protein per day which he needs for good feet and to rebuild muscle, strong tendons and just about every other working part.
So you go to the feed store to find him something to boost his protein - only to find that all the labels list protein as "%" with none of them telling you how many grams of protein there are in a "serving", and most likely the feeding directions say to "feed to condition" or "to maintain body weight".
But you brought your calculator and wrote down the simple formula we used to figure the hay protein, so you can now calculate that -
12% protein is 12/100x453.6 = 54.4 grams protein per pound
14% protein is 14/100x453.6 = 63.5 grams protein per pound
16% protein is 16/100x453.6 = 72.5 grams protein per pound
20% protein is 20/100x453.6 = 90.7 grams protein per pound
You also realize that he'll need at least 3 lbs/day of the 12% protein feed but only 2.3 lbs of the 16% protein feed so, with other things being equal, the 16% feed may be more cost effective.