Rumen microbes break down degradable protein to small peptides, amino acids, and ammonia. These products can in turn be used by rumen microbes to produce microbial protein that can be digested by the cow in the small intestine. Microbial protein is an excellent quality protein, unfortunately not enough can be produced to supply the requirement for the high producing dairy cow. Therefore, undegradable protein must be available to make up the difference between what the cow requires and what the microbial protein supplies. Feeds vary in their ability to supply undegradable protein and quality will vary by source and depend on digestibility and amino acid composition. If the undegradable protein is indigestible or has a poor amino acid profile it will be of little value. A recent study published in the Journal of Dairy Science (Vol. 82, pages 2585-2595) compared high (18%) and low (15%) crude protein content of diets with and without ruminally protected methionine and a source of unprotected lysine. These two amino acids are generally considered to be first limiting for milk protein production. All diets were composed of alfalfa hay, corn silage, corn grain, soybean hulls, whole cottonseeds, soybean meal, corn gluten meal, animal fat, and a mineral-vitamin mix. Whole soybeans were added only to the diets that contained no supplemental methionine and lysine. Corn distillers grains were in diets with supplemental methionine and lysine. There were no differences in dry matter intake or milk production due to level of protein or amino acid supplementation when diets were fed to early lactation cows averaging over 100 pounds of milk per day. There did tend to be more milk protein concentration and yield when amino acids were included. Higher protein resulted in greater urea production in the liver and as a result more blood urea indicating a less efficient use of dietary protein. The final conclusion was that in early lactation the level of dietary protein was less important for milk protein synthesis than was amino acid profile. This study supports the idea that for milk protein (casein) production proper combinations of protein sources are important especially in early lactation. Commercial supplementation of amino acids should be evaluated based on economic merit relative to other sources of amino acids. These other sources could be but are not limited to fish meal, blood meal, and mechanically extracted soybean meals.

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