Effects of a fermented cotton straw-apple pomace mixture on growth performance, rumen microbial community, and metabolome in beef cattle

Introduction The utilization of agricultural by-products as feed plays a significant role in reducing resource waste and promoting sustainable development of the livestock industry. This study investigated the effects of replacing corn silage with a fermented cotton straw-apple pomace mixture in beef cattle diets. Methods Twenty beef cattle were randomly assigned to two groups: a control (CON) group fed a basal diet and a treatment (TRE) group fed a diet in which corn silage was replaced by the fermented mixture. We assessed growth performance, rumen fermentation parameters, fibrolytic enzyme activities, bacteria community structure, and metabolite profiles. Results Compared to the CON group, the TRE group showed reductions in average daily gain (ADG) and dry matter intake (DMI) by 25.42 and 18.79%, respectively ( p < 0.05). The concentrations of ruminal volatile fatty acids (VFAs) were also significantly lower (ranging from 9.63 to 17.01% reduction; p < 0.05). The activities of cellulase, hemicellulase, and cellobiohydrolase were significantly decreased in the TRE group (by 13.22, 38.80, and 13.66%, respectively; p < 0.05). The fermented mixture also altered the rumen microbial composition: the relative abundances of Anaeroplasma and Pyramidobacter were higher in the TRE group, whereas those of Anseongella, Holdemania, and Acetoanaerobium were lower. Non-targeted metabolomics analysis revealed significant changes in the rumen metabolite profile of cattle fed the fermented mixture; notably, ferulic acid concentrations were significantly higher in the TRE group than in the CON group. Spearman correlation analysis indicated that Anseongella was significantly positively correlated with ADG, while Pyramidobacter was significantly negatively correlated with ADG and with the activities of cellulase, hemicellulase, cellobiohydrolase. Anaeroplasma was negatively correlated with those enzyme activities and was also significantly associated with ferulic acid and many other differential metabolites. Discussion In conclusion, replacing corn silage with the fermented cotton straw-apple pomace mixture reduced ADG, VFA concentrations, and ruminal fibrolytic enzyme activities in beef cattle; these effects may be related to changes in specific rumen bacteria and metabolites.