Fumarate improves growth performance and fiber digestibility alongside rumen microbial and hydrogen metabolic remodeling in starch-rich-fed Hu sheep

Fumarate is widely regarded as an alternative ruminal electron acceptor and a precursor for propionate formation. Under starch-rich conditions, however, basal fermentation may already be biased toward propionate formation. Under these conditions, the metabolic routes through which fumarate exerts its effects remain poorly defined. We evaluated how fumarate supplementation affected growth performance, nutrient digestibility, rumen fermentation and microbial functional profiles in Hu lambs fed a starch-rich diet. Twenty male Hu lambs were randomly assigned to the control (CON) or fumarate-supplemented (FUM) treatment. We measured growth performance, apparent nutrient digestibility, ruminal VFA concentrations, pH, and dissolved H₂ and CH₄, and characterized the rumen metagenome. The FUM treatment increased final body weight and ADG and improved feed efficiency. The FUM treatment also enhanced the apparent digestibility of OM, NDF, ADF and EE. Ruminal pH and concentrations of butyrate and dissolved H₂ were higher with the FUM treatment. Propionate concentration and the acetate/propionate ratio were unaffected, whereas dissolved CH₄ tended to increase. Metagenomic analysis showed that Fibrobacterota, Methanobacteriota and Ciliophora were more abundant under the FUM treatment. CAZyme families associated with starch utilization decreased in abundance, whereas those involved in fiber degradation increased. Hydrogen metabolism profiling showed no enrichment of fumarate reduction or other competing H₂-consuming pathways. In contrast, genes encoding [FeFe]-A3 hydrogenases and methanogenesis-associated mcr genes increased in abundance. Collectively, fumarate supplementation improved fiber digestibility and growth performance in Hu lambs fed a starch-rich diet. These effects were associated with a higher ruminal pH and greater microbial potential for fiber degradation, together with enrichment of [FeFe]-A3 hydrogenases and methanogenesis-associated genes but no detectable enrichment of fumarate-reduction or other alternative electron-disposal pathways.