149 Using Carbon Emissions, Oxygen Consumption, and Energy Retention to Estimate Dietary ME Intake By Beef Steers

Abstract Eight cross-bred beef steers (initial BW = 241 ± 4.10 kg) were used in a 77-d feeding experiment to determine if ME intake can be determined from carbon emissions, oxygen consumption, and energy retention estimates. Steers were housed in a pen equipped with individual feed bunks and animal access was controlled by Calan gates and trained for 2 wk before starting the experiment. After training, steers were then randomly assigned to 1 of 3 treatments as a multiple of ME intake (MEx) at maintenance (7.43 Mcal of ME/d): 1.1 (1.1M, n = 3), 1.4 (1.4M, n = 2), and 1.8 (1.8M, n = 3). Diets fed were long-stemmed wheat hay (Triticum aestivum L.; 15% CP, 64% TDN) plus a daily supplement with 1.025 kg of alfalfa(Medicago sativa L.)/wheat middling pellets (15% CP) delivered via an automated head-chamber system (GreenFeed, C-Lock, Inc., Rapid City, SD) to measure CO2 and CH4 emission, and O2 consumption. On d 1, 14, 35, 56 and 77 of the experiment, BW was recorded after a 17-h fast and used to calculate retained energy (RE) and adjust feeding rates of the diets. Retained energy by the steers was calculated using the NASEM (2016) procedures. Heat production (HP) was calculated from gas emission and consumption estimates and urinary N output [Nurine = 5.91 x dietary CP (%) – 21.52, g/d] using the Brouwer (1965) equation. Intake of ME was calculated by adding HP and RE. Total DMI linearly increased (P < 0.01) with MEx (4.10, 5.44, and 7.41 kg/d for 1.1M, 1.4M, and 1.8M, respectively) and steer ADG (0.12, 0.29, and 0.65 kg, respectively) increased linearly (P < 0.01) with increasing MEx. Carbon dioxide emissions (3,846, 4,106, and 4,468 g/d, respectively) and O2 consumption (3,406, 3,514, and 3,964 g/d, respectively) increased linearly (P ≤ 0.01) with increasing MEx, but CH4 did not differ (P > 0.41; 162, 170, and 170 g/d, respectively). Energy retention (0.32, 0.72, and 1.89 Mcal/d, respectively) and HP (11.35, 11.80, and 13.24 Mcal/d, respectively) by steers increased (P < 0.01) linearly with MEx. Hence, the predicted ME intake increased linearly (P < 0.01) from 1.1M to 1.8M (11.67, 12.52, and 15.13 Mcal/d, respectively). These results indicate that measurements of CO2 and CH4 emissions, and O2 consumption with an automated head-chamber system can be used to provide reasonable estimates of ME intake and HP for grazing ruminants.