Investigation of guanidino acetic acid and rumen-protected methionine induced improvements in longissimus lumborum muscle quality in beef cattle.

This study examined the impact of dietary guanidino acetic acid (GAA) and rumen-protected methionine (RPM) on beef quality in Simmental bulls. For 140 days, forty-five bulls (453.43 ± 29.05 kg) were randomly divided into control (CON), 0.1% GAA (GAA), and 0.1% GAA + 0.1% RPM (GAM) groups with 15 bulls in each group and containing 3 pen with 5 bulls in each pen. Significant improvements in eye muscle area, pH, redness (a*) value, and crude protein (CP) content of longissimus lumborum (LL) muscles were observed in the GAA and GAM groups (P < 0.05). Conversely, the lightness (L*) value, drip loss, cooking loss, and moisture contents decreased (P < 0.05). Additionally, glutathione (GSH) and glutathione peroxidase (GSH-P) concentrations of LL muscles in GAM were higher (P < 0.05), while malondialdehyde (MDA) content of LL muscles in GAA and GAM groups were lower (P < 0.05). Polyunsaturated fatty acids (PUFA) profiles were enriched in beef from GAM group (P < 0.05). The addition of GAA and RPM affected the expression of genes in LL muscle, such as HMOX1, EIF4E, SCD5, and NOS2, which are related to hypoxia metabolism, protein synthesis, and unsaturated fatty acid synthesis-related signaling pathways. In addition, GAA and RPM also affected the content of a series of metabolites such as L-tyrosine, L-tryptophan, and PC (O-16:0/0:0) involved in amino acid and lipid metabolism-related signaling pathways. In summary, GAA and RPM can improve the beef quality and its nutritional composition. These changes may be related to changes in gene expression and metabolic pathways related to protein metabolism and lipid metabolism in beef.