Although most people can quote the percent protein in their feed, and manufacturers make a point of it, this particular piece of information is almost worthless. The horse's protein requirement is in grams per day, not a percentage in the grain. The percentage itself - e.g. 10% protein = 10 grams protein per 100 grams of feed - tells you nothing without knowing how much of it the horse eats. You also need to know what the horse's requirement is and how much protein is coming from the rest of the diet. See My Example: Let's take a 500 kg (1100 pound) horse in moderate work to see how this works. According to the NRC recommendations, that horse needs 1.536 grams of protein per kg of body weight so 768 grams/day. If the horse is eating nothing but a 9% protein hay, 10 kg (22 lbs) a day, his protein intake is 900 grams. If that same horse is eating 2.5 kg (5.5lbs) of a 10% protein grain and 5 kg (11 lbs) of hay his protein intake is only 700 grams/day, close but deficient. There is also new research showing that the ideal protein intake for working horses is 2+ grams/kg of body weight, making the diet considerably more deficient. If you upgrade the horse above to 10% protein hay, intake would be 1000 grams on the hay only diet and his needs would be met. What about if you change the grain to 14% protein, same amounts fed of grain and hay? The 5 kg of hay would provide 500 grams of protein and the 14% protein grain at 2.5 kg (5.5 lbs) gives 350 for a total of 850 grams versus the 1000 needed. Let that sink in. If you feed nothing but a 10% protein hay you can meet even the higher end of estimated protein needs for a horse in moderate work. If you cut hay in half and substitute 5.5 lbs of a 14% protein grain, you will be deficient. The take home message here is that the % protein in your grain is no guarantee the diet is adequate, even if a "high" protein like 14%. That 14% protein grain has 1.4 times more protein than a 10% protein hay, but 2.5 or even 3.0 times more calories. When you feed grain, if you don't want weight gain you have to cut back on the amount of hay fed. In many cases that is actually the reason for feeding grain - to control the amount of hay needed. If you feed 1 lb of grain instead of 3 lbs of hay to equal calories, that grain must also have 3 times more protein than the hay to keep protein intake the same. The only way to know if your diet is adequate is to know the requirement and the protein level in everything fed. For adult horses at maintenance or low work, it's actually not often an issue if hay quality is good. For more active horses, pregnant, breeding, lactating and growing horses, it's an entirely different story because their protein needs are much higher. Don't assume a "high" protein grain can get the job done. Do the math!