Bill – thanks for this. Determining how much protein to feed takes some math. Before I do that, I want to remind everyone that proteins are made of 20 amino acids. Of these, ten need to be eaten, while the other ten the horse actually can make. Therefore these ten are called “essential.” Hay does not provide all of the ten essential amino acids. You can feed all the hay a horse can eat in a day, and they will still be deficient in some amino acids and, therefore, deficient in many proteins.

Here is the math. First, find the crude protein (CP) of the hay you feed. Let’s say it’s 12% CP. Assuming the horse eats 1.5% of his body weight and weighs 1100 pounds, that’s 16.5 pounds of hay. 12% of this is CP which = 1.98 pounds. If the bioavailability is 50%, the actual protein he is eating is half of 1.98 pounds or .99 pounds. Now for the conversion to grams. 1 pound = 454g, so .99 lb = 449.46 g

The requirement of protein for horses is similar to humans: 0.5 to 1.0 g per pound of body weight. An 1100 pound horse will need a range of 550 to 1100 grams of protein per day. Feeding 16.5 pounds of 12% CP hay will only give about 450g of good-quality protein (not high-quality with all the essential amino acids).

The hiccup in this is using grams and pounds together. Converting everything to metric – 1.5% of 500kg = 7.5kg hay. 12% of this is 0.9kg CP. 50% of this is 0.45kg or 450g of absorbed protein.

Now for the main issue. The NRC requirement for protein is too low, or they are confusing crude protein with actual protein absorbed and used by the horse. Instead of 630g, I think they are saying 315g. In humans, the NRC has recommended this low amount of protein (about 0.33g/pound BW) because, according to the people I listen to, the value they recommend is the amount needed to keep us alive. However, to thrive, research shows a minimum of 0.5g per pound of BW (or 1g per kg) up to 1g per pound (2g/kg).

Does this make sense?