PS2-1. Ex vivo and in vivo Methods Comparisons for Daily Methane Production Estimates in Commercial Beef x Dairy Steers

Abstract The objective of this study was to evaluate ex vivo and in vivo methane (CH4) measurement approaches in beef × dairy steers. Ruminally cannulated steers (n = 5; initial BW = 443 ± 15 kg) were co-housed with commercial beef × dairy steers (n = 67; BW = 272 ± 24 kg) across two pens (34–35 hd/pen) at a commercial feedyard. All steers were limit-fed a common total mixed ration (1.26 Mcal/kg NEg) twice daily for 56 d. Greenfeed Automated Head Chamber Systems (AHCS; C-Lock Inc., Rapid City, SD) were placed in each pen to quantify greenhouse gas emissions from both cannulated and non-cannulated steers. AHCS CH4 data were compared to ex vivo gas production estimates determined using an ANKOM RF gas production system (ANKOM Technology, Macedon, NY). On d 56, rumen fluid was collected from each cannulated steer. Rumen fluid was added to fermentation flasks containing a ground sample of the TMR offered to the pen, with gas production recorded cumulatively over 48 h and collected via attached foil Tedlar bags for CH4 compositional analysis by gas chromatography. Ex vivo total gas production was converted to daily CH4 output by scaling batch culture measurements based on either the volume of rumen fluid added—extrapolated to the rumen digesta capacity of cannulated animals as determined by manual evacuation 4 h post-feeding—or the amount of substrate added, extrapolated to pen-level dry matter intake. Data were analyzed in R (v4.4.2) using linear models, with method effects tested via Tukey-adjusted pairwise comparisons. Five daily CH4 quantification approaches were compared: 1) filtered AHCS pen-level data (≥10 visits/animal; 143.9 g/d), 2) all viable AHCS data for pen regardless of visits (139.2 g/d), 3) AHCS measurements from cannulated steers (39.3 g/d), 4) ex vivo substrate-based estimates (145.8 g/d), and 5) ex vivo rumen-capacity estimates (149.5 g/d). Mean CH4 production differed among methods (P 0.05). Estimates derived from ex vivo rumen fluid, ex vivo substrate extrapolation, and pen-level AHCS measurements did not differ from one another (P 0.10). However, CH4 measured in vivo from cannulated steers using AHCS was lower (P 0.05) than all other methods. The consistency among pen-level AHCS measurements and ex vivo approaches suggests that ex vivo extrapolation methods provide methane estimates comparable to AHCS measurements in commercial cattle under field conditions. Lower emissions measured using AHCS from cannulated animals likely reflect incomplete sealing at the cannula interface rather than methodological inaccuracy. The findings support the use of co-housed cannulates and ex vivo gas production systems as a lower cost tool for estimating methane production in commercial beef × dairy cattle.