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 CH 4 yield from cattle and sheep. Accordingly, ruminants were categorised into 3 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 CH 4 yield (g/kg DM intake [ DMI ] and g/kg digestible OM intake [ DOMI ]; CH 4 /DMI and CH 4 /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 CH 4 emissions depended on the metric of CH 4 emission, being consistently stronger (i.e., higher R 2 ) for CH 4 /DOMI compared to CH 4 /DMI for the 3 ruminant classes. For CH 4 /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, CH 4 /DMI was related to ATTDOM (R 2 = 0.35, slope = -0.146), NDF content (R 2 = 0.68, slope = 0.014), and CP content (R 2 = 0.65, slope = -0.021). For other cattle, CH 4 /DMI was only related to CP content (R 2 = 0.18, slope = -0.008), and for sheep, tendencies for relationships in opposite directions than expected were detected between CH 4 /DMI and both ATTDOM (R 2 = 0.39, slope = 0.064) and NDF content (R 2 = 0.46, slope = -0.007). For CH 4 /DOMI, the relationships with forage quality characteristics were statistically significant for the 3 ruminant classes, and always in the expected directions, likely due to comparable (i) shifts in ruminal fermentation and consequently H 2 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 CH 4 emissions by improved forage quality differs between the 3 ruminant classes for CH 4 /DMI, being effective in lactating dairy cattle, effective to a lesser extent in other cattle, and not effective or increasing CH 4 /DMI in sheep, likely driven by differences in extent of fermentation and feeding level between these ruminants. For CH 4 /DOMI, improved forage quality effectively reduced enteric CH 4 across dairy cattle, other cattle, and sheep, with ATTDOM and CP content negatively related and NDF content positively with CH 4 /DOMI.