Feedlot efficiency implications on greenhouse gas emissions and sustainability.

The term sustainable has many meanings, but in agriculture it generally refers to some balance between environmental, social, and economic goals. The objective of this project was to quantify inputs and outputs to assess the sustainability implications of 2 feedlot cattle management systems: Never Ever 3 (NE3) and a conventional (CON) system using metabolic modifiers. Angus-cross steers (n=104) were stratified by BW (337 kg ± 17) and randomly assigned to 4 pens per treatment group. The NE3 cattle received no feed additives or implants, whereas CON were implanted with 100 mg of trenbolone acetate and 14 mg of estradiol benzoate on d 1 and 70, and were additionally fed monensin [330 mg/(animal·d)] and tylosin phosphate [90 mg/(animal·d)] in their ration throughout the course of the study, and ractopamine hydrochloride at 254 mg/(animal·d) for the last 29 d on feed. Cattle were shipped on a constant average pen weight basis (596 kg ± 32 BW). The CON cattle had greater ADG (1.81 vs. 1.35 kg, P < 0.01) and were on feed fewer days (146 vs. 188 d, P < 0.01) than the NE3 cattle. No significant differences were observed in HCW (P = 0.072) or dressing percentage (P=0.62) between treatments (P > 0.05); however, CON carcasses averaged larger ribeye area (87 vs. 80 cm(2), P < 0.01), greater Warner-Bratzler shear force measurement (WBSF; 3.46 vs. 3.19 kg, P < 0.01), and smaller USDA marbling score (5.4 vs. 6.2, P < 0.01), and less backfat thickness (1.64 vs. 1.84 cm, P < 0.05) and yield grade (3.38 vs. 3.95, P < 0.01) than NE3 carcasses. Overall, CON cattle consumed 393 kg less DM in the feedlot (1,250 vs. 1,643 kg; P < 0.05). No treatment effects were observed for daily methane (CH(4); P=0.62) or nitrous oxide (N(2)O; P=0.7) emissions per steer. Assuming a constant emission rate on a DMI basis throughout the course of the feedlot trial, CON feedlot management resulted in a 31% decrease in emissions per finished steer compared with NE3 management. Expressing CH(4) emissions on a carbon dioxide equivalent (CO(2)-eq) basis revealed a 1.10-kg CO(2)-eq difference per kilogram BW gain (5.02 kg of NE3 vs. 3.92 kg of CON) between the 2 feedlot management systems. Although the metabolic modifiers resulted in additional costs for the CON treatment group, the cost per kilogram of feedlot BW gain was significantly less ($1.12/kg vs. $1.35/kg; P < 0.05) than NE3. Both production systems satisfied some sustainability criteria, although neither concurrently fulfilled all of the environmental, social, and economic goals of agricultural sustainability.