Effects of Italian ryegrass silage-based TMR on rumen fermentation, growth performance, blood metabolites, and bacterial communities of growing Hanwoo heifers

This study utilized Italian ryegrass silage (IRGS) - based TMR as feedstuff and evaluated its effects on rumen fermentation, growth performance, blood parameters, and bacterial community in growing Hanwoo heifers. Twenty-seven Hanwoo heifers (body weight, 225.11 ± 10.57 kg) were randomly allocated to three experimental diets. Heifers were fed 1 of 3 treatments as follows: TMR with oat, timothy, and alfalfa hay (CON), TMR with 19% of IRGS (L-IRGS), and TMR with 36% of IRGS (H-IRGS). Feeding high levels of IRGS (H-IRGS) and CON TMR to heifers resulted in a greater molar proportion of propionate in the rumen. The impact of different TMR diets on the BW, ADG, DMI, and FCR of Hanwoo heifers during the growing period did not differ (<italic>p</italic> &gt; 0.05). Furthermore, the blood metabolites, total protein, albumin, AST, glucose, and total cholesterol of the heifers were not affected by the different TMR diets (<italic>p</italic> &gt; 0.05). In terms of rumen bacterial community composition, 264 operational taxonomic units (OTUs) were observed across the three TMR diets with 240, 239, and 220 OTUs in CON, L-IRGS, and H-IRGS, respectively. IRGS-based diets increased the relative abundances of genera belonging to phylum <italic>Bacteroidetes</italic> but decreased the abundances of genus belonging to phylum <italic>Firmicutes</italic> compared with the control. Data showed that <italic>Bacteroidetes</italic> was the most dominant phylum, while <italic>Prevotella ruminicola</italic> was the dominant species across the three TMR groups. The relative abundance of <italic>Ruminococcus bromii</italic> in the rumen increased in heifers fed with high inclusion of IRGS in the TMR (H-IRGS TMR). The relative abundance of <italic>R. bromii</italic> in the rumen significantly increased when heifers were fed H-IRGS TMR while<italic> P. ruminicola</italic> increased in both L-IRGS and H-IRGS TMR groups. Results from the current study demonstrate that the inclusion of Italian ryegrass silage in the TMR is comparable with the TMR containing high-quality forage (CON). Thus, a high level of IRGS can be used as a replacement forage ingredient in TMR feeding and had a beneficial effect of possibly modulating the rumen bacterial community toward mainly propionate-producing microorganisms.