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Limit-feeding of a high concentrate diet has been proposed as an effective method for improving feed efficiency and reducing total manure output of dairy heifers; meanwhile the effects of this method on hindgut microbiota are still unclear. This study investigated the effects of a wide range of dietary forage:concentrate ratios (F:C) on the fecal composition of bacteria and archaea in heifers using next-generation sequencing. Four diets with different F:C (80:20, 60:40, 40:60, and 20:80) were limit-fed to 24 Holstein heifers, and the fecal fermentation parameters and bacterial and archaeal communities were investigated. With increasing dietary concentrate levels, the fecal dry matter output, neutral detergent fiber (NDF) content, and proportion of acetate decreased linearly ( P  < 0.01), while the fecal starch content and proportions of propionate, butyrate, and total branched-chain volatile fatty acids (TBCVFAs) were increased ( P  ≤ 0.05). An increased concentrate level linearly increased ( P  = 0.02) the relative abundance of  Proteobacteria , and linearly decreased ( P  = 0.02) the relative abundance of  Bacteroidetes  in feces. At the genus level, the relative abundance of unclassified Ruminococcaceae and  Paludibacter  which may have the potential to degrade forage decreased linearly ( q  ≤ 0.02) with increasing dietary concentrate levels, while the relative abundance of  Roseburia  and  Succinivibrio  which may be non-fibrous carbohydrate degrading bacteria increased linearly ( q  ≤ 0.05). Some core microbiota operational taxonomic units (OTUs) also showed significant association with fecal VFAs, NDF, and/or acid detergent fiber (ADF) content. Meanwhile, the relative abundance of most detected taxa in archaea were similar across different F:C, and only  Methanosphaera  showed a linear decrease ( P  = 0.01) in high concentrate diets. Our study provides a better understanding of fecal fermentation parameters and microbiota under a wide range of dietary F:C. These findings support the potential for microbial manipulation by diet, which could enhance feed digestibility and relieve environmental problems associated with heifer rearing.

Effect of Limit-Fed Diets With Different Forage to Concentrate Ratios on Fecal Bacterial and Archaeal Community Composition in Holstein Heifers