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Amino acid nutrition of lactating dairy cows: Applying the concept of Ideal Protein
The dairy industry has been slowly developing and adopting the ideal protein concept. Research beginning over 40 years ago has established Lys and Met as generally the two most limiting AA, and that the sequence and extent of their limitation is determined by the amount and AA composition of rumen escape feed protein. The first computerized model to predict directly the “content” of AA in duodenal protein was the AA submodel of Rulquin et al. (1998), which used the INRA (1989) PDI system to predict flows of protein fractions. The second was the AA submodel in NRC (2001). While different approaches were used for predicting the AA composition of duodenal protein, both models provided for a true integration of the AA submodel with the protein model and allowed for minimal biases of prediction. Dose-response relationships were subsequently developed for both models that related changes in predicted concentrations of Lys and Met in digestible protein to measured changes in milk protein production. This was the first step in defining the ideal profile of absorbed AA in lactating cows. By using these models and balancing for the model-determined optimal levels of Lys and Met in digestible protein, field nutritionists have been able to take the first step in applying the concept of ideal protein in ration formulation. After its release, the NRC model was used, in conjunction with published experiments with measured yields of milk and milk components, to arrive at initial estimates of Lys and Met requirements. The resulting regression equations have been implemented in Formulate2, a ration formulation program that is fully compliant with the NRC 2001 model. Based on field observations, this has been a huge step forward in balancing diets for Lys and Met. By integrating data from experiments in which individual and combinations of AA were infused postruminally in dairy cows, Canadian researchers have proposed ideal profiles of absorbed AA for lactating cows when using the NRC model. Researchers are debating how to further develop and validate the concept of ideal protein for lactating cows. There are several reasons for this and these will be discussed. While progress in developing the concept of ideal protein has been slow, it is known that an improved balance of digestible AA increases efficiency of utilization of AA for milk protein production and reduces need for rumen escape feed protein.
Keywords: Ideal protein, lactating dairy cows, amino acids