IntroductionProtein nutrition is critical for high production efficiency of lactating dairy cows because it impacts their performanceand the environment. Sufficient dietary protein is required to optimize production while an excess has negative effectson the environment, primarily when excreted as urea in urine. The major protein sources used in western areas of NorthAmerica include high quality alfalfa hay, whole cottonseed or cottonseed meal, dried distillers grains with solubles (DDGS)and canola meal (CM). Due to the variable quality and high price of alfalfa hay, and the presence of secondary compounds(i.e., tannins and gossypol) in cottonseed, their inclusion levels in dairy rations are limited. Therefore, use of CM and DDGSas major supplemental protein sources is currently very high in many US dairy rations.