Diet dominates, but biochar contributes: longitudinal effects of biochar supplementation on the beef cattle microbiome

Biochar is widely promoted as a ruminant feed additive for methane mitigation and microbiome modulation, but microbiome evidence from realistic farm settings remains limited and may be confounded by diet transitions. We conducted a seven-month, on-farm longitudinal study of UK Sussex beef cattle in Kent, monitoring two herds (n=25 each) from November 2023 to June 2024 as management shifted from barn-fed fodder to pasture forage. One herd received ad libitum hardwood biochar via a trough, while the other served as a contemporaneous control. Faecal DNA was profiled by 16S rRNA (V3–V4) amplicon sequencing and analysed using alpha- and beta-diversity metrics, including multivariable association testing (MaAsLin3) that factored in potential confounding covariates such as diet. Redundancy analysis indicated that herd of origin and diet explained significant variance in community structure, while biochar supplementation explained comparatively little variance; however, biochar exposure was associated with statistically significant compositional separation from controls across overlapping post-supplementation timepoints. Changes in archaeal reads, while unreliable, suggested that Methanosphaera was more prevalent in cattle fed biochar. Biochar did not drive a sustained increase in richness and was associated with taxon-specific enrichment of selected gut microbial lineages. However, the association with biochar on many of these taxa were flipped when considering the change in diet between forage and fodder feeding. These findings highlight the need for further biochar research under realistic farm conditions that explicitly account for dietary transitions.