Red blood cells in all mammals carry oxygen to the tissues by way of a protein called hemoglobin.  Hemoglobin binds and carries oxygen and is bound to a molecule of iron.  Anemia is a decrease in the number of red blood cells or a decrease in the percentage of red blood cells compared to plasma, called the hematocrit.  Since red blood cells carry oxygen, the more red blood cells a mammal has in the circulation that are carrying oxygen, the greater ability a horse will have to perform athletic endeavors such as jumping or barrel racing.  In fact, many people have tried for years to give horses blood builders to increase the number of red blood cells to give their horse an edge when racing.  Many owners and trainers of horses want their red blood count checked to determine if their horse has an optimum number of red blood cells so they can carry more oxygen to the tissue and therefore run faster and longer.  However, it is not that simple. 

The horse is unique among domestic animals in that they store up to one-third of their red blood cells in their spleen and they can release these cells with intense exercise.  So you could check the number of red blood cells or hematocrit of a resting horse, and then exercise the horse and after intense exercise the red blood cell count would be much higher.  The amount of increase will be related to the amount of exercise, so checking a resting horse’s red blood cell count is not necessarily going to indicate their ability to perform.  If the red blood cell count is significantly low, then the horse needs to be checked out further for anemia.  However, trying to judge a horse’s ability to perform athletic endeavors by looking at a resting red blood cell count is unlikely to be effective.