The seventh revised edition of the U.S. National Research Council (NRC) Nutrient Requirements of Dairy Cattle feeding guide, considered an international reference, came out in 2001.The eighth revised edition was published in 2021. The first change pertains to a name: The NRC is now the National Academies of Sciences, Engineering and Medicine (NASEM).
Aside from this name change, what is new in this revised edition in relation to dairy cow protein feeding? The revised edition of the NASEM guide is based on the use of research results published in the literature, including research funded by Dairy Farmers of Canada. A potential benefit of the revised edition is an improved efficiency in the use of protein and amino acids, resulting in a reduction in the cost of rations and a smaller dairy farm environmental footprint.
We will discuss this in two parts: protein and essential amino acid (EAA) supply, and prediction of milk protein production and EAA recommendations (to be covered in a future article).
Protein supply
Digested protein generating a new supply of EAA, called metabolizable protein (MP), comes from the digested fraction of rumen-undegraded dietary protein and microbial protein. However, the protein that enters the first segment (or duodenum) of the small intestine comes from three sources.
1. Rumen-undegraded dietary protein
This is the dietary protein that has escaped degradation from ruminal microorganisms. As in the NRC (2001) version, the rumen-undegraded dietary protein of each ingredient is estimated using fractions A, B and C, determined according to ruminal degradation kinetics. Degradation kinetics is obtained by incubating small bags containing the ingredient studied in the rumen of cows for different periods of time.
In this latest edition, two applications of these fractions differ from the NRC (2001) version. Rumen-undegraded dietary protein is now calculated according to fixed ruminal passage rates (kp) for Fraction B, one for concentrates and one for forages, whereas this passage rate was variable in the NRC (2001) version. Fraction A now contributes to the calculation of MP, whereas previously it was considered to be completely degraded in the rumen.
It is important to mention that the intestinal digestibility of dietary protein has been revised. Rumen-degraded dietary protein is estimated by the difference between the total protein ingested and the rumen-undegraded dietary protein.
2. Microbial protein
This protein comes from the growth of microorganisms. After developing in the rumen, which serves as a fermenter, the microorganisms are expelled from the rumen and form the microbial protein. The new model predicts microbial protein by combining estimated rumen-degraded dietary protein and the amount and type of energy (starch, fibre) available in the rumen, all calculated using new equations. The NRC (2001) version used rumen-degraded dietary protein and total tract digestible energy for this calculation.
3. Endogenous protein
This protein is made by the cow itself and then recirculated in the gut lumen. For example, sloughed cells (dead and detached from the walls) and enzymes of the gastric compartment enter into this category. Contrary to the NRC (2001) version, MP no longer includes endogenous protein entering the duodenum, considered as recycling.
EAA supply
This estimate has been completely revised. Net EAA supply is now referred to as metabolizable EAA, in reference to MP. The NRC (2001) version used a correction equation based on the duodenal flux of EAA in undegraded dietary protein. NASEM calculates EAA uptake directly by multiplying each protein fraction entering the duodenum by its EAA composition. The EAA composition of the microbial protein was determined. The EAA composition of the ingredients constituting the undegraded dietary protein and their digestibility have been revised. As with MP, metabolizable EAA no longer include endogenous protein entering the duodenum.
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Dairy Farmers of Canada (DFC) invests in scientific research to foster innovation in the Canadian dairy sector. DFC supports research initiatives that benefit all Canadian dairy farmers and works in collaboration with its members and other sector partners to address priorities outlined in the National Dairy Research Strategy. The goals of this strategy are to increase farm efficiency and sustainability, enhance animal health, care and welfare practices, and strengthen the role of dairy in human nutrition and health, as well as in sustainable diets. Visit dairyfarmersofcanada.ca/en/dairy-research for more information.
