Power Down The Protein: What A Horse Eats Impacts His Air Quality Sponsored by: Though changing what a horse eats may seem an odd thing to adjust for air quality, research has shown that reducing the amount of protein in a horse's diet can protect his respiratory health. Protein is broken down into nitrogen in the horse's small intestine and then excreted as urea, which becomes ammonia in a horse's stall. Ammonia can irritate nose and lung tissue, causing excess mucus production and respiratory issues like heaves. Drs. Jessie Weir-Chouinard, Hong Li, Lori Warren, Erica Macon, and Carissa Wickens created a study to decide how much protein impacted ammonia levels. Researchers fed nine horses forage-based diets with three different levels of protein, the highest being 12 percent.