70. Understanding the Emissions of Feedlot Beef Cattle with Divergent Residual Methane Emissions.

Abstract Residual methane emission (RME) has been proposed as a novel metric to identify cattle that emit less methane independently of their feed intake and body weight. This study investigated how divergence in RME classification influences feed intake, growth rate, feed efficiency, feeding behavior, enteric gas emissions, and apparent nutrient digestibility in crossbred beef cattle consuming a forage-based diet. A total of seventy-eight growing beef cattle (steers n = 19; heifers n = 59; initial body weight 463 ± 57 kg; mean age 551 ± 22 d) were blocked by body weight and housed in four pens. Each pen was equipped with three automated feed intake bunks (Vytelle SENSE, Lenexa, KS) and one GreenFeed Emission Monitoring system (C-Lock Inc., Rapid City, SD) for quantifying individual dry matter intake (DMI) and enteric gas emissions over an 84-d period. Residual methane emission was calculated for each animal as the residual from a multiple regression model of methane production on DMI and mid-test metabolic body weight (BW0.75). Based on these residuals, animals were classified into low (n = 25), medium (n = 27), or high (n = 26) RME categories (+/- 0.5 SD from mean). Data were analyzed using a mixed model in JMP Pro (v.16; SAS Institute Inc., Cary, NC) with RME class as a fixed effect, pen and sex as a random effect. Significance was declared at P ≤ 0.05, and trends were considered at P ≤ 0.10. As expected, RME classification did not affect (P > 0.50) DMI and metabolic BW. Additionally, RME did not affect ADG (P = 0.39), indicating that methane divergence occurred independently of feed intake and growth. Measures of feeding behavior, including bunk visit duration and eating rate, were similar among RME classes (P > 0.90); however, cattle in the low RME group tended to visit the bunk less frequently (P = 0.08). Enteric gas emissions differed markedly across RME classifications. Low RME animals produced approximately 20% less daily methane (163 vs. 205 g/d; P > 0.001), exhibited 20% lower methane yield (17.9 vs. 22.4 g/kg DMI; P > 0.001), and 21% lower methane per metabolic BW (1.51 vs. 1.92 g/kg BW0.75; P > 0.001) compared with high RME counterparts. Carbon dioxide output was reduced by 8% (P = 0.002), and hydrogen emissions tended to be lower (P = 0.06) in low RME animals. Across animals, RME was strongly correlated with methane production (r = 0.81), methane yield (r = 0.63), and methane intensity (r = 0.88). Overall, cattle identified as low RME emitted substantially less methane while maintaining similar feed intake, and growth performance. These results reinforce RME as a biologically relevant trait for identifying low methane-emitting, efficient beef cattle, offering a promising strategy for reducing the environmental footprint of ruminant production without compromising productivity.