Animal Science Department


Date of this Version



The Professional Animal Scientist 22 (2006):307–317


Based on applications of the 1996 National Research Council (NRC) Nutrient Requirement of Beef Cattle (2000 update) model at our laboratories and on use of the model to evaluate published literature, it is our assessment that the NRC model (Level 1) represents an advancement in nutrient requirements of grazing cattle. We evaluated 8 published studies (35 treatment observations) to compare observed changes in body condition score (BCS) to those predicted by the NRC model, and to develop criteria to help with application of the model to range cattle nutrition. When in vitro organic matter disappearance (IVOMD) of grazed diets was converted to DE [DE = (1.07 × IVOMD) − 8.13] and used as a proxy for TDN, there were no differences in observed vs. predicted BCS change (P = 0.44). A critical component of the 1996 NRC model is the incorporation of the metabolizable protein system. An accurate estimate of microbial efficiency is key to application of the metabolizable protein system. We propose the use of an equation to adjust microbial efficiency for diets less that 65% TDN. The NRC model should be applied to 30-d time intervals if animal requirements or forage quality are changing rapidly. Accurate inputs for stage of gestation, days into lactation, breed, milk production, dietary TDN, and microbial efficiency are necessary for the model to yield accurate results. Work is needed to define and model microbial efficiency, nitrogen recycling, and the quality of diets selected by grazing livestock in various environments.