Cows do not have a crude protein requirement. yet, many rations still put too much weight on dietary crude protein, says Gabriella Varga, dairy nutritionist at PennStateUniversity. Shift the focus to metabolizable protein instead. Optimizing the synthesis of this “true” protein source in the cow has its rewards. (For more details, please see “Target true protein” in the July 2008 issue.)

Ask your nutritionist how much “true” protein is getting into your rations.

Study up on protein degradability

Before you inquire about the emphasis that metabolizable protein is getting in your rations, familiarize yourself with some of the basic terms associated with protein use by the cow.

Rumen degradable protein (RDP) and rumen undegradable protein (RUP) are two components of dietary crude protein. Rumen microbes take the degradable protein and fermentable carbohydrates that are fed to the cow and turn them into microbial protein. Undegradable protein is just that — the protein fed to the cow that bypasses the rumen.

Together, this microbial protein and undegradable protein form metabolizable protein, Varga explains. The rumen channels this protein to the small intestine — the site of “true” protein absorption. This is where cows absorb amino acids and convert them into milk and milk protein.

Your nutritionist can optimize this process. Many ration-balancing programs use equations to calculate or predict how much metabolizable protein your cows need to make milk.

Now it’s time to find out if this is taking place on your farm. Ask your nutritionist the questions to the right.

Q: Besides crude protein, what other protein needs of my cows are you using to balance the ration?

Emphasis on rumen degradable protein, undegradable protein and metabolizable protein is a good answer. You also want to hear that your nutritionist is giving attention to bypass-protein sources that are highly digestible in the small intestine, says Dwight Roseler, a Land O’ Lakes Purina Feed dairy nutrition consultant in the Great Lakes region.

Q: What is the level of rumen degradable protein in the diet? 

Aim for an RDP level of 9.5 percent to 10 percent of ration dry matter, Varga says. Discuss early-lactation versus late-lactation dry matter intake levels, too. This impacts RDP levels.

Q: What is the level of rumen undegradable protein in the diet?

Aim for a range of 5 percent to 7 percent of diet dry matter. Use a highly digestible bypass protein with a proper amino acid profile, Roseler says.

Q: What sources of amino acids are you using to balance the diet?

Sources of protected amino acids will vary, Roseler says. Use a bypass methionine source. Pay attention to dry matter intake levels. Well-managed herds with high dry matter intake may not need a lot of additional bypass amino acids.

Remember, too, that carbohydrates, especially starch and fiber, help make microbial protein, so balance for them as well, Varga says.

The next step

Are you satisfied with your nutritionist’s answers, or could your ration stand room for improvement when it comes to metabolizable protein? If so, focus on efforts that improve the supply of metabolizable protein and its amino acid profile. You won’t be disappointed with the results.