Effects of Level of Intake of a 50% Concentrate Pelleted Diet on Digestion and Energy Utilization By Katahdin Wethers

Nine mature Katahdin (KAT) wethers (70 ± 1.5 kg initial BW) were used in a crossover experiment to evaluate effects on digestion and energy utilization of levels of feed intake being employed in studies addressing the maintenance energy requirement with limited nutrient intake. A 50% concentrate pelleted diet composed of 20.0% ground alfalfa, 29.1% cottonseed hulls, 9.0% cottonseed meal, 20.0% ground corn, 13.0% wheat middlings, 5.0% pelleting agent, and 3.9% other ingredients was fed near the ME requirement for maintenance (Control; DMI = 44.4 g/kg BW^0.75) and at 55% of this level (Restricted; DMI = 24.4 g/kg BW^0.75). Periods were 4 wk in length, with 3 wk for adaptation, measures in the final week when situated in metabolism cages, and 2 d for gas exchange measurement via a head-box respiration calorimetry system. Apparent total tract digestibilities of DM (65.8 and 73.9%; SEM = 1.92), OM (76.3 and 81.7%; SEM = 1.48), CP (72.1 and 78.5%; SEM = 1.70), NDF (32.3 and 49.0%; SEM = 3.34), and gross energy (64.7 and 73.0%; SEM = 1.97) were greater (P < 0.05) for Restricted than for Control intake. Expressed in MJ/d, quantities of energy in urine (0.94 and 0.72; SEM = 0.320) and ruminally emitted methane (1.02 and 0.76; SEM = 0.085) were greater for Control vs. Restricted intake (P < 0.05), but as a percentage of DE they tended to be greater for Restricted intake (urine: 8.1 and 10.8%, SEM = 3.52, and P = 0.056; methane: 9.0 and 11.1% for Control and Restricted intake, respectively, SEM = 0.75, and P = 0.096). As a consequence, ME intake as a percentage of gross energy intake did not differ (P = 0.301) between treatments (53.5 and 57.3% for Control and Restricted intake, respectively; SEM = 2.88). The difference in heat energy (447 and 379; SEM = 15.1) was less than that in ME intake (395 and 225 kJ/kg BW^0.75 for Control and Restricted intake, respectively; SEM = 21.4). In conclusion, restricted feed intake had marked influences on digestibility, although effect on metabolizability was tempered by changes in urinary and methane energy, the former presumably impacted by lean tissue mobilization. Based on the magnitude of difference between ME intake and heat energy with restricted intake, lower heat energy and less tissue mobilization would be expected with longer periods.