268 Determination and classification of growing steers according to residual methane emissions.

Abstract Animal classification according to residual methane emission (RCH4) allows the identification of low-methane (CH4) emitters without affecting animal performance. RCH4 is the difference between observed and predicted CH4 emissions. The objectives of this study were to determine the relationship between animal performance and gas flux variables in estimating CH4 emissions in growing steers, and to determine whether growing steers change their RCH4 classification during the growing period. In the backgrounding phase, 83 steers [200 ± 28 kg of initial body weight (BW)] were fed a total mixed ratio (TMR) with 46% neutral detergent fiber (NDF) and 0.86 Mcal/kg DM of net energy of gain (NEg) for 60 d. During the finishing phase, 47 steers received (542 ± 38 kg of initial BW) and 37 steers did not receive (471 ± 69 kg of initial BW) a growth-promoting implant and were fed a TMR with 17.5% NDF and 1.32 Mcal/kg DM of NEg for 80 d. Feed intake and gas flux were determined daily using Smartfeed and Greenfeed technology. Steers were weighed every 3 wk during each phase to determine average daily gain. In both phases, estimated CH4 emission was calculated according to the individual metabolic BW and dry matter intake (DMI, ECH4BD), carbon dioxide (CO2) production (ECH4CD), and the CO2:CH4 ratio (ECH4CC). Then, RCH4 emissions were calculated as the observed CH4 production and ECH4BD (RCH4BD), ECH4CD (RCH4CD), and ECH4CC (RCH4CC). Based on the quartile distribution, steers were classified as low, medium, and high emitters. The relationship between RCH4 production during the backgrounding and finishing phases from implanted and non-implanted steers was evaluated using the chi-square test and regression analysis. Animal growth performance and gas flux variables from steers in each category were analyzed using a mixed model. DMI and BW, CO2 production, and CO2:CH4 ratio explained 57, 90, and 18% of CH4 variability during the backgrounding phase, while between 24 to 49, 60 to 66, and 57 to 72% during the finishing phase, respectively. On average, 64% of steers changed the classification using different RCH4 classifications. No relation existed between individual RCH4 classification during the backgrounding and finishing phases. Steers classified according to RCH4BD and RCH4CD classification had similar animal growth performance and lower (p< 0.05) CH4 production than those classified as high RCH4 emitters. Using the RCH4CC classification, low-emitter steers had lower (p< 0.05) animal performance and gas production than those classified as high-emitters. In conclusion, the classification of growing steers using RCH4BD and RCH4CD results in animals with lower CH4 production without affecting animal growth performance. However, RCH4 classification should be conducted in each phase because RCH4 classification during the finishing cannot be practiced using the RCH4 classification during the backgrounding phase.