Abstract
Residual expressions of enteric emissions favour a more equitable identification of an animals methanogenic potential compared to traditional measures of enteric emissions. The objective of this study was to investigate the effect of divergently ranking beef cattle for residual methane emissions (RME) on animal productivity, enteric emissions and rumen fermentation. Dry matter intake (DMI), growth, feed efficiency, carcass output and enteric emissions (Greenfeed emissions monitoring system) was recorded on 294 crossbred beef cattle ((steers = 135 and heifers = 159; mean age 441 days (SD = 49); initial body weight (BW) of 476 kg (SD = 67)) at the Irish national beef cattle performance test centre. Animals were offered a total mixed ration (77% concentrate and 23% forage; 12.6 MJ ME/kg of DM and 12% CP) ad libitum with emissions estimated for 21 days over a mean feed intake measurement period of 91 days. Animals had a mean daily methane emissions (DME) of 229.18 g/d (SD = 45.96), methane yield (MY) of 22.07 g/kg of DMI (SD = 4.06), methane intensity (MI) 0.70g/kg of carcass weight (SD = 0.15) and RME 0.00 g/d (SD = 0.34). RME was computed as the residuals from a multiple regression model regressing DME on DMI and BW (R 2=0.45%). Animals were ranked into three groups viz. high RME (>0.5 SD above the mean), medium RME (±0.5 SD above/below the mean) and low RME (>0.5 SD below the mean). Low RME animals produced 17.6 and 30.4% less (P<0.05) DME compared to medium and high RME animals, respectively. A ~30% reduction in MY and MI was detected in low vs. high RME animals. Positive correlations were apparent amongst all methane traits with RME most highly associated with (r=0.86) DME. MY and MI were correlated (P<0.05) with DMI, growth, feed efficiency and carcass output. High RME had lower (P<0.05) ruminal propionate compared to low RME animals and increased (P<0.05) butyrate compared to medium and low RME animals. Propionate was negatively associated (P<0.05) with all methane traits. Greater acetate:propionate ratio was associated with higher RME (r=0.18; P<0.05). Under the ad libitum feeding regime deployed here, RME was the best predictor of DME and only methane trait independent of animal productivity. Ranking animals on RME presents the opportunity to exploit inter animal variation in enteric emissions as well as providing a more equitable index of the methanogenic potential of an animal on which to investigate the underlying biological regulatory mechanisms.
Effect of divergence in residual methane emissions on feed intake and efficiency, growth and carcass performance, and indices of rumen fermentation and methane emissions in finishing beef cattle
