In Vitro Incubations Do Not Reflect In Vivo Differences Based on Ranking of Low and High Methane Emitters in Dairy...

Area of Science:

• Ruminant nutrition and metabolism
• Environmental science and sustainability
• Microbial ecology

Background:

• Accurate in vitro methods are needed to rank dairy cows by methane (CH4) emissions for targeted mitigation strategies.
• The GreenFeed system (GF) allows in vivo CH4 measurement, but its replication in vitro is challenging.
• Understanding fermentation balance is crucial for assessing CH4 production implications.

Purpose of the Study:

• To evaluate if in vivo dairy cow CH4 emitter ranking (low vs. high) can be replicated using an in vitro automated gas system.
• To assess the implications of this in vitro replication on fermentation balance.
• To investigate the effects of diet and cashew nutshell liquid (CNSL) on CH4 production and fermentation in vitro.

Main Methods:

• Seven pairs of low and high CH4-emitting cows were selected based on residual CH4 production from the GF system.
• Rumen fluid from each pair was incubated separately in an in vitro gas production system using two substrates (grass silage vs. barley-based) with or without CNSL.
• CH4 production, volatile fatty acids (VFA), and microbial structure (16S rRNA gene sequencing) were analyzed.

Main Results:

• In vivo ranking of cows was poorly replicated in vitro; no significant differences in animal performance or pre-incubation rumen fermentation were found between low and high emitters.
• Diet composition significantly affected fermentation: a 50% barley (SB) diet increased gas production and predicted CH4, promoting propionate and reducing acetate compared to a grass silage (S) diet.
• Cashew nutshell liquid (CNSL) consistently decreased predicted CH4 and stoichiometric CH4 (CH4VFA), reducing total gas production per VFA, irrespective of emitter group.

Conclusions:

• The in vitro gas production system, using rumen fluid, did not effectively replicate the in vivo ranking of dairy cows based on CH4 emissions.
• Diet composition and the addition of CNSL demonstrated more consistent and significant impacts on in vitro CH4 production and rumen fermentation than individual cow emitter status.
• Further research is needed to refine in vitro systems for accurate prediction of individual animal CH4 emissions and to optimize mitigation strategies using feed additives like CNSL.