Effects of a Guanidinoacetic Acid–Cysteamine Compound Additive on Rumen Metabolism, Fermentation Parameters, and Microbial Communities in Dairy Bulls

This study aimed to determine the effects of dietary supplementation with a compound additive containing guanidinoacetic acid (GAA) and cysteamine on rumen metabolism, fermentation, and the microbiota of fattening Holstein bulls. Sixty Holstein dairy bulls (294.33 ± 3.42 kg) were divided into three groups (n = 20). Each treatment comprised three independently fed pens (n = 3 pens per treatment), and one bull was sampled from each pen for the rumen measurements. Group A served as the control group, whereas Groups B and C received a concentrated supplement containing 1 kg/t and 2 kg/t of a compound additive (GAA combined with cysteamine), respectively, over 90 days. Kyoto Encyclopedia of Genes and Genomes identified five key pathways. Specifically, indole-3-acetaldehyde, 2-amino-3-methoxybenzoic acid, 2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexene-1-carboxylate, and cholesterol glucuronide were upregulated, whereas 4-fumarylacetoacetic acid and pseudouridine 5′-phosphate were downregulated. Compared with those in the control group, acetic acid, butyric acid, total volatile fatty acids (VFAs), propionic acid, and microbial crude protein (MCP) increased, whereas pH, acetic acid/propionic acid (A/P) ratio, and NH3-N did not differ. At the genus level, Weissella abundance increased, Methanobrevibacter abundance decreased, and Xylanibacter and Ruminococcus abundances increased in the low-supplementation group (1 kg/t). Methanobrevibacter correlated negatively with VFAs, MCP, and NH3-N but correlated positively with A/P. Weissella and Ruminococcus showed the opposite trend. Increasing the inclusion level of the compound additive improved rumen metabolism, fermentation, and the microbial environment. This ultimately reduced stress and enhanced rumen health in fattening Holstein bulls.