Forage characteristics as drivers of variation in enteric methane emission in cattle and sheep: a meta-analysis

A meta-analysis was performed to determine whether improved forage quality is equally effective at reducing CH₄ yield from cattle and sheep. Accordingly, ruminants were categorised into three classes: lactating dairy cattle, other cattle (i.e., beef cattle and non-lactating dairy cattle), and sheep. Data from 24 studies (109 treatment means) met the selection criteria for inclusion in the meta-analysis. The relationship between CH₄ yield (g/kg DM intake (DMI) and g/kg digestible OM intake (DOMI); CH₄/DMI and CH₄/DOMI, respectively) and forage quality characteristics (apparent total tract organic matter digestibility (%; ATTDOM), NDF content (g/kg DM), and CP content (g/kg DM)) was determined using a mixed model univariate regression procedure. The relationship between forage quality characteristics and CH₄ emissions depended on the metric of CH₄ emission, being consistently stronger (i.e., higher R²) for CH₄/DOMI compared to CH₄/DMI for the three ruminant classes. For CH₄/DMI, substantial differences were observed due to differences in extent of ruminal fermentation and associated processes as well as differences in feed intake level (kg/d and g/kg metabolic BW). For lactating dairy cattle, CH₄/DMI was related to ATTDOM (R² = 0.35, slope = -0.146), NDF content (R² = 0.68, slope = 0.014), and CP content (R² = 0.65, slope = -0.021). For other cattle, CH₄/DMI was only related to CP content (R² = 0.18, slope = -0.008), and for sheep, tendencies for relationships in opposite directions than expected were detected between CH₄/DMI and both ATTDOM (R² = 0.39, slope = 0.064) and NDF content (R² = 0.46, slope = -0.007). For CH₄/DOMI, the relationships with forage quality characteristics were statistically significant for the three ruminant classes, and always in the expected directions, likely due to comparable (i) shifts in ruminal fermentation and consequently H₂ availability, (ii) increased microbial efficiency and consequently less volatile fatty acids per unit of organic matter (OM) fermented, and (iii) methanogen activity between cattle and sheep. It is concluded that the effectiveness of reducing enteric CH₄ emissions by improved forage quality differs between the three ruminant classes for CH₄/DMI, being effective in lactating dairy cattle, effective to a lesser extent in other cattle, and not effective or increasing CH₄/DMI in sheep, likely driven by differences in extent of fermentation and feeding level between these ruminants. For CH₄/DOMI, improved forage quality effectively reduced enteric CH₄ across dairy cattle, other cattle, and sheep, with ATTDOM and CP content negatively related and NDF content positively related to CH₄/DOMI.