Nutrient Requirements for Dairy Cattle of the National Research Council versus some commonly used ration software

The first edition of the Nutrient Requirements of Dairy Cattle was published by the National Research Council (NRC) in 1945. The current document is the sixth revised edition, published in 1989, and it ap peals that we are a few years from another edition being in print. Software designed to evaluate and formulate rations for dairy cattle commonly determine nutrient requirements using the NRC as a standard. However, the generation of new knowledge in dairy nutrition occurs more rapidly than the release of the NRC publication, and the developers of the software often modify the requirements based on more recently published research, geographical peculiarities, or factors not explicitly considered by NRC. The first step in evaluating or formulating rations is the prediction of dry matter intake (DMI). The primary variables used by NRC to predict DMI are body weight (BW) and fat-corrected milk (FCM) yield; however, developers of software programs often use different equations based on personal preference, availability of research data with given equations, and incorporation of other factors in addition to BW and FCM yield. The additional factors are included to provide a more dynamic estimation of DMI and, therefore, reduce the difference between predicted and actual DMI. Nutrients required for maintenance, lactation, and growth must be consumed in adequate quantities (e.g., kilograms or calories), but the dietary concentration of nutrients for a given animal group may differ because of DMI. Even when nutrients are fed above the requirements, dietary concentrations of nutrients may be important in some situations to minimize the risk of underfeeding caused by variability in the nutrient composition of feedstuffs and to account for interactions of certain nutrients (e.g., minerals). New research discoveries need to be incorporated into ration formulation strategies promptly, and the strategies used for ration formulation need to be dynamic.