By Vicky Carson
Crude protein is just that – crude! For far too long, we've been using CP as a measure of forage and overall diet quality and a tool to produce milk yield.
It estimates of protein content of feed or milk. But it isn't the best or most precise measure of what cows utilize, and leaves you vulnerable to lower production and waste. Here's why:
CP measures all nitrogen in a sample, including true protein (protein made of amino acids) and non-protein nitrogen (nitrogenous compounds like urea). So your sample of second-cut haylage can have a high CP percentage on a dry matter basis, but not provide the cow or her rumen bugs with amino acids.
In forages it's generally the result of highly soluble protein. Some soluble protein is needed by rumen microbes. But when it's in excess, that N leaves the rumen as ammonia. Ammonia travels to the liver and is metabolized to urea and urea is either recycled (via saliva) or excreted in urine and/or milk.
Very little of that N is used for milk protein synthesis. It actually costs you and your cows energy to deal with that excess.
What to use instead
We should be balancing diets using metabolizable protein. MP is the measure of absorbable amino acids after losses due to digestion and absorption.
Metabolizable amino acids are available to the cow for productive purposes like milk yield, milk protein synthesis, reproduction, immune function, and growth. Diet evaluation software programs, like the Cornell Net Carbohydrate and Protein System and Cornell Penn Miner, calculate MP from rumen undegraded protein in feeds and MP from rumen microorganisms.
These mumbo-jumbo terms are important. Herds with highest milk yields and milk true protein yields have diets balanced for MP from bacteria greater than 51% of total MP.
Carson and husband Steve partner in Harkdale Farms of Newbury, Vt. She's also a professor at Vermont Technical College. This article was originally published in the American Agriculturalist.