Knowledge in the NRC pubs
Feed characterization using CHO and protein fractions in feeds, DMI prediction, rumen models, growth requirement, aminoacid, MP system, and computer models
60 refs
Net energy systems
Feed characterization: CHO and protein
Metabolizable protein and ruminaly degraded protein
1200 (beef) refs
1900 (dairy) refs
The next question is how did we get to the current requirements?  The NRC provides recommendations most often used as a basis for ration formulation systems.  Having been involved with 6 NRC committees over the past 20 years, it is my observation that these committees do an excellent job of summarizing the accumulated data and making recommendations, based on published data.   The weight of the evidence indicates we have a lot of accumulated knowledge of cattle nutrient requirements.  The 1963 Beef NRC had 60 references; the 1996 beef NRC had about 1200 references and the 2001 Dairy NRC had about 1900 references. The 1966 Dairy NRC had a short paragraph about crude protein requirements and 10 references; the 2001 Dairy NRC had 42 pages and 770 references on protein requirements.

I’ll do a quick review of the major improvements that I have seen and used in the field that improved accuracy of predicting requirements.  1) The introduction of the NE system in 1970 and 1971 improved the accuracy in predicting energy requirements for maintenance, growth, and lactation and the useful energy in feeds to meet those requirements.  2) In the 80’s, improved methods to describe feed carbohydrate and protein fractions were developed. 3) In 1989 and 1996 the metabolizable protein system was introduced to improve the accuracy in meeting ruminal N requirements and undegraded diet protein to supplement microbial protein.  4)  Accumulated knowledge led to considerable refinement in requirements in 1996 and 2001 NRC: a) Description of feeds by carbohydrate and protein fractions, (b) Equations to predict feed energy values from feed analysis, (c) improved accuracy in predicting DMI, (D) rumen models that predict ruminal N, microbial protein production and escape of undegraded feed protein and amino acids, (D) systems to accurately predict growth requirements for cattle with different mature sizes, (e) amino acid requirements for growth and milk, and (f) computer models to integrate and apply the accumulated knowledge.