Residual methane emission determination and classification of growing steers during the backgrounding and finishing phases.

This study aimed to assess the relationship between animal growth performance and gas flux variables in predicting methane (CH4) emissions, and to determine the consistency of residual CH4 emission (RME) classification in growing steers during the backgrounding and finishing phases. A total of 83 Angus steers (approximately 8 mo of age and 200 ± 27.9 kg of initial body weight [BW]) were enrolled during the backgrounding phase. During the finishing phase, 36 steers did not receive a growth-promoting implant, while 47 steers were implanted with a growth-promoting implant. Individual animal growth performance, dry matter intake, and gas flux (CH4 and carbon dioxide [CO2] production and oxygen [O2] consumption) were determined in both phases. The relationship between observed CH4 emission, animal growth performance, and gas flux variables was evaluated using regression analysis. Residual methane emissions was determined using metabolic BW and dry matter intake (i.e., RME_MD) and CO2 production (i.e., RME_CO2) as regressors for estimating CH4 production. Steers were ranked in ascending order according to their RME_MD and RME_CO2. Then, steers were classified as low, medium, and high emitters based on the quartile distribution. Gas flux and animal growth performance were then analyzed using a completely randomized design for each phase. The relationship between RME classification during backgrounding and the subsequent classification during finishing was analyzed using regression and Fisher's exact tests. Animal growth performance explained between 26% and 50% of the CH4 production, while CO2 production and O2 consumption explained between 47% and 90% of CH4 production during the backgrounding and finishing phases, respectively. When transitioning from backgrounding to a finishing phase, 38% of the steers maintained their initial classification. Steers divergently classified according to RME_MD or RME_CO2 did not differ (P > 0.10) in growth performance during either backgrounding or finishing phases. However, steers with low RME had lower (P ≤ 0.001) CH4 production than those classified as high RME. In conclusion, RME_MD and RME_CO2 identified growing steers with lower CH4 emissions without affecting animal growth performance during the backgrounding and finishing phases. However, RME classification should be conducted in each phase because growing steers were re-ranked by 62% when changing from the backgrounding to the finishing phase.