269 The relationship between metabolic gas emissions and performance in Angus Bulls and heifers.

Abstract Methane (CH₄) production is a moderately heritable trait. However, selection based on methane production may reduce economically relevant traits, which decreases methane intensity and overall profitability. The objective of this project was to evaluate the relationship between gas emissions and animal performance characteristics. Individual animal feed intake and body weights (BW) were recorded daily using SmartFeed and SmartScale (C-Lock Inc., USA) from 181 Angus bulls and 113 Angus heifers during a 64-d and 60-d performance test, respectively. Performance traits were composed of metabolic BW, average daily gain (ADG), dry matter intake (DMI), and residual feed intake (RFI). During the test period, 2 GreenFeeds (GF; C-Lock Inc.) were used to collect metabolic gas fluxes. Animals were rotated every 8 to 21 d through pens with the GF, so all animals were exposed to GF during the performance test. Only animals with at least 20 GF visits were included in the analysis, resulting from data from 127 bulls and 80 heifers. Daily averages (g/d) of CH₄, carbon dioxide (CO₂), and oxygen (O₂) were 167 ± 26.7, 7111 ± 638, and 5488 ± 483 for bulls; and 179 ± 22.7, 6681 ± 673, and 4282 ± 478 for heifers. In addition to CH₄ production, CH₄ intensity (g of CH₄ per kg of ADG), CH₄ yield (g of CH₄ per kg of DMI), and residual methane emissions (RME) were evaluated. RME was computed as the residuals from a regression of CH₄ production on DMI and metabolic BW. Individual metabolic heat production (HP; kcal) was also calculated using the Brouwer Equation. Average CH₄ intensity (CH4 g/kg ADG), CH₄ yield (CH4 g/kg DMI), and RME (g/d) were 110 ± 42, 21.1 ± 4.4, and 0 ± 26.6 for bulls; and 202 ± 1754, 17 ± 5.7, and 0 ± 18.1 for heifers. Gas fluxes were correlated with greater metabolic BW (from 0.10 to 0.63), ADG (from 0.03 to 0.61), DMI (from 0.07 to 0.60), and RFI (from 0.07 to 0.32). In bulls, CH₄ intensity was negatively correlated with BW (-0.35), ADG (-0.79) and DMI (-0.35), but not correlated with RFI. In both bulls and heifers, CH₄ yield was negatively correlated with BW (-0.28 and -0.39), ADG (-0.33 and -0.25), DMI (-0.61 and -0.92), and RFI (-0.47 and -0.83). RME was favorably correlated with RFI in heifers (0.41). For bulls and heifers, HP was positively correlated with BW (0.60 and 0.42), ADG (0.57 and 0.28), DMI (0.50 and 0.49), and RFI (0.13 and 0.28). The findings of this study suggest that selection for methane reduction may favor feed efficiency in Angus cattle.