TECHNICAL NOTE: Estimating beef-cattle forage demand: Evaluating the animal unit concept

The objective of this experiment was to evaluate the effect of BW and physiological status of a beef animal on forage intake. The experiment was repeated over 2 yr with 6 replications of 3 treatments per year: cow-calf pair (CCP, BW = 629 kg), nonlactating cow (NLC, BW = 503 kg), and yearling steer (YS, BW = 305 kg). The CCP was treated as one unit, with the sum of cow BW and calf BW comprising CCP BW. Calves averaged 42 d of age and 73 kg at the start of the experiment each year. Animals were housed in individual pens and fed grass hay harvested from subirrigated meadow (11% CP) in quantities sufficient for ad libitum intake. Intake of DM, OM, DM that disappeared in vitro, and NDF were greatest (P < 0.01) for CCP, intermediate for NLC, and least for YS. As a percentage of BW, the CCP had greater (P < 0.01) intake of DM, OM, DM that disappeared in vitro, and NDF than did both the NLC and YS, which were not different (P > 0.05) from each other. When expressed as a percentage of metabolic BW (BW0.75), intake of DM, OM, DM that disappeared in vitro, and NDF were greatest (P < 0.01) for CCP, intermediate for NLC, and least for YS. Results indicate that intake differences among cattle of different physiological states should be considered when calculating forage demand for stocking rate or feeding purposes.