Friday, April 29, 2011

The Models Used to Formulate AA Balanced Rations

Basic knowledge of amino acid (AA) nutrition for lactating cows has significantly advanced in the last 50 years. However, only recently has that wisdom been harnessed into models that can be used on a day to day basis.

In the U.S., the three basic models currently used are NRC 2001, CNCPS and CPM. Over time, they have all evolved and changed to include improved equations that predict the cow’s responses with better accuracy. In 2001, NRC published its latest version, the last version of CPM (v.3) was released in 2003, and CNCPS released its latest version (v.6) in 2007. Each model has its own set of equations (i.e. “biology”) to predict the amino acid profile of the metabolizable protein (MP).

Each model also has its own methodology to evaluate the nutritional "quality" of the feed’s proteins. These differences have a direct impact on the predicted grams of MP and individual AAs the cow has available to fulfill her requirements.

The protein of any feed is partitioned into fractions A, B or C in the NRC model, or into fractions A, B1, B2, B3 and C in the CPM and CNCPS models:

Besides some differences in how these fractions are defined in each model, CPM and CNCPS have different degradation rates (Kd) for each fraction. In general, the Kds for the B1 and B2 fractions are lower for CNCPS v.6 versus CPM v.3. Hence, each model will provide a different prediction of MP supply and the AA profile of the MP.

In the next Technical mail we will discuss the impact of these differences and how we interpret each model’s output to optimize the AA utilization for maximal milk output.

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