Replacing corn-silage in total mixed rations for fattening bulls with dry components: Effects on rumen health, the buccal and faecal microbiome and the plasma metabolome

The exchange of feed components in a diet requires a concise ration-adjustment to obtain a ration that is rumen-safe, as well as energy- and protein-balanced. The growth rates achievable with a straw- and concentrate-based ration, supplemented with by-products ( DRY ) were previously shown to equal those obtained with an isocaloric and isonitrogenous conventional corn silage-based ration ( CONVL ) throughout a 9-month fattening period in 24 Simmental bulls. Differences in fibre composition and particle size may nevertheless affect gut health and result in subclinical alterations. We therefore investigated rumen health, the buccal and faecal microbiota using 16S ribosomal RNA gene sequencing, and the plasma metabolome. Bulls fed the DRY diet showed decreased rumen motility and increased rumen wall thickness, indicating long-term effects on the rumen. The plasma metabolomic profiles from 51 and 63 weeks of age (28 and 40 weeks on feed) exhibited specific alterations in the DRY-fed bulls, with increased levels of trimethylamine N-oxide ( TMAO ) and methionine sulfoxide, and decreased levels of hippuric acid and 3-indolepropionic acid ( 3-IPA ) suggesting potential oxidative and metabolic stress. In addition, lipid metabolism was altered in DRY-fed bulls, with changes in phosphatidylcholines, cholesterol esters, and triglycerides. Buccal swab samples collected at week 63 of age showed lower alpha diversity and fewer amplicon sequence variants in DRY-fed than in CONVL-fed bulls, whereas faecal alpha diversity did not differ between diets. Beta-diversity ordination showed more pronounced diet-associated separation in buccal swab samples than in faecal samples. Differential abundance analysis showed less butyrate-producing Butyrivibrio , Pseudobutyrivibrio and Lachnospiraceae as well as intestinal mucosa-maintaining Akkermansia, in the DRY-fed bulls than in the CONVL group, following the differences in the intake of physically effective NDF ( peNDF ). These findings suggest that while both total mixed rations supported similar growth performance, the DRY diet affected rumen and metabolic health and altered the microbial composition, indicating that the animals’ capacity to adapt to certain diets might reach its limits.