Beef Species Symposium: difficulties associated with predicting forage intake by grazing beef cows

The current NRC model to estimate DMI is based on a single equation related to metabolic size and net energy density of the diet; this equation was a significant improvement over previous models. However, observed DMI by grazing animals can be conceptualized by a function that includes animal demand, largely determined by metabolic or linear size, physiological state, genetics, or any combination of these. Even in the database used to generate the current NRC equation, DMI by cows is poorly predicted at the extremes. In fact, across a wide range of actual DMI, predicted DMI is rather flat, indicating an insensitivity of prediction, so the model requires further refinement. A broad-based database was developed that includes pasture and confinement studies with growing, nonlactating, and lactating cattle. New equations are presented for consideration in the new model. It was found that the premise behind earlier NRC equations based on diet digestibility and BW are sound but that for cows, additional drivers based on milk production or calf performance were stronger than BW. Future models should be based on multiple variables, including functions for physiological state, animal suitability to the environment, and activity to modify the predicted DMI. Further, the model could possibly account for imbalances of protein to energy, particularly as they relate to ruminal function. Further, the issue of how reference data were collected (pen vs. pasture) and how the methods or constraints influence DMI must be evaluated. Overall, the new NRC model needs to be more robust in its ability to account for the wide variation in the environment, dietary characteristics, and metabolic demands.