# Rumen Microbial Composition and Fermentation Variables Associated with Methane Production in Italian Simmental Dairy Cows

## TL;DR

This study identifies differences in the gut microbiome and fermentation traits of dairy cows that produce high versus low methane emissions, suggesting that altering the microbiome could help reduce methane.

## Contribution

The study provides insights into how rumen microbial composition and fermentation variables are linked to methane emissions in dairy cows.

## Key findings

- High methane emitters had less diverse microbiomes dominated by methanogens, while low emitters had more diverse communities with hydrogen-utilizing bacteria.
- Volatile fatty acid profiles and VOC levels differed significantly between high and low methane emitter groups.
- Neutral detergent fiber digestibility and valeric acid concentration were higher in high methane emitters.

## Abstract

This study aims to determine the differences in ruminal and fecal microbiota between lactating dairy cows that are considered high and low methane emitters. The emissions of 48 dairy cows were measured using a hand-held methane sensor during feeding. To assess the differences in methane emissions, the 12 highest and 12 lowest emitters were selected, and rumen fluid and fecal samples were taken in order to analyze the volatile fatty acids and the volatile organic compounds. As expected, the grams of methane emitted per kg of dry matter intake were higher in the high emitters than in the low emitters. The high emitters showed a less diverse microbiome dominated by methanogens, whereas low emitters had a more diverse community dominated by hydrogen-utilizing bacteria that reduce hydrogen availability for methanogenesis.

The study investigated differences in ruminal and fecal microbiota composition, fermentation traits, and volatile organic compounds (VOC) in Simmental dairy cows classified as high (HME) or low (LME) methane emitters. Methane emissions from 48 cows were quantified using the Laser Methane Smart portable gas detector. The 12 animals with the highest and lowest emissions were selected and assigned to the HME and LME groups, respectively, balanced for body weight, days in milk, and body condition score. Rumen fluid and fecal samples were analyzed for pH, ammonia, volatile fatty acids (VFA), VOC, and microbiota composition. As expected, CH4 emissions were significantly higher in HME than in LME cows (22.5 vs. 13.2 g/kg DMI; 16.9 vs. 8.4 g/kg FCM). The neutral detergent fiber digestibility was higher in HME cows (51.4% vs. 47.9%). The valeric acid concentration and the acetate-to-propionate ratio were significantly higher in HME cows (3.53 vs. 3.31). The VOC profiles significantly differed between groups in both feces and rumen fluid. The microbiota analysis revealed a significant difference between groups at the order and genus levels (Bray–Curtis dissimilarity). The Shannon index was higher in LME cows (2.08 vs. 1.95). HME cows exhibited a higher abundance of Methanosphaera and Methanobacteriales. Overall, the results indicate that re-shaping the rumen microbial community can play a key role in reducing methane emissions, strengthening the case for microbiome-driven approaches and offering insights that can support mitigation strategies across dairy production systems.