Protein is usually the most expensive feed ingredient. Secondly, there is the risk of an improper protein to calorie ratio. In young horses, this can lead to decreased feed intake and reduced growth. Another concern is the body's need to get rid of the nitrogen from protein, resulting in greater water intake and excretion of urea. An ammonia smell may be noticed in poorly ventilated stables when too much protein is fed.
Young foals are susceptible to ammonia, which can cause pneumonia.
Extra protein can aggravate kidney failure and liver disease problems in older horses. Frederick Harper, Extension Horse Specialist, University of Tennessee.