PSVIII-19 - Assessing methane and carbon dioxide production in beef cows across diverse foraging conditions

Abstract: Beef cattle grazing across more than 40M ha of Canada's grasslands is economically significant yet contributes to methane (CH₄) emissions. Accurately measuring CH₄ emissions across diverse environments presents substantial challenges. Our study investigated CH₄ and carbon dioxide (CO₂) production in three-year-old pregnant crossbred beef cows (n = 30) across different phases of the beef production cycle, including in drylot and while grazing on native rangeland, in Western Canada's Aspen Parkland region using the GreenFeed Emissions Monitoring System (GEM).

During the January to March drylot phase, cows' enteric CH₄ and CO₂ production were monitored for 63 days in consort with feed efficiency testing while consuming a mixed oat-barley silage diet. Following this, cows were categorized into three distinct groups based on the standard deviation (SD) of CH₄ yield (g kg⁻¹ DMI): Low ( < 0.5 SD; n = 11), Medium (±0.5 SD; n = 10), and High ( >0.5 SD; n = 9). Post-calving, cows and calves transitioned to native pastures for CH₄ and CO₂ assessment across three distinct foraging conditions: high-quality, high-quantity forage in summer (SUM, 50 d); moderate-quality, high-quantity forage in September (SEP; 22 d); and finally, low-quality, low-quantity forage in October (OCT; 22 d).

We hypothesize that ranking cows based on their CH₄ yield (g kg⁻¹ DMI) in drylot settings may have the potential to reflect their CH₄ production (g/day) during grazing conditions, even without feed intake data. Data were analyzed using the PROC MIXED procedure of SAS to examine CH₄ production among cows categorized by their assigned ranking. Spot CH₄ and CO₂ measurements totaled 1,242, 1,145, and 1,205 for the SUM, SEP, and OCT, production phases, respectively. Average daily visits to GEM units were 1.4 ± 0.1, 1.84 ± 0.1, and 1.96 ± 0.1 for the corresponding phases.

While High CH₄-ranked cows had methane production similar to Low CH₄-ranked cows (234.8 ± 8.2 vs. 235.0 ± 6.0 g/day, respectively), the Medium group had significantly higher methane production (260.5 ± 6.2 g/day; P = 0.008) than the Low and High CH₄ groups. Furthermore, significant interactions were observed between CH₄ ranking groups and CH₄ production during the grazing phase (P = 0.035). Cows in the Medium CH₄ group emitted higher levels of CH₄ compared to the High group in SUM (288.2 ± 9.3 vs. 247.0 ± 14.1 g/day) and to the Low group in SEP and OCT (276.5 ± 6.6 vs. 238.1 ± 6.3, and 216.7 ± 7.2 vs. 191.8 ± 7.5 g/day, respectively).

In conclusion, the drylot CH₄ ranking may hold promise in predicting outcomes for both Low and Medium CH₄-ranked groups during grazing phases. However, High CH₄-ranked cows had lower methane production, likely influenced by grazing-induced changes in feed intake and individual feeding behaviors, prompting further exploration.