Effects of Dietary NFC/NDF and Allicin Supplementation on Serum Electrolytes, Nitrogen and Phosphorus Excretion, and Fecal Microbiota in Beef Bulls

This study evaluated the effects of dietary non-fiber carbohydrate to neutral detergent fiber ratio (NFC/NDF) and allicin supplementation on serum electrolytes, urinary biochemical indicators, nitrogen (N) and phosphorus excretion, and fecal microbiota in beef bulls. Forty 12-month-old beef bulls were assigned to a 2 × 2 factorial design based on dietary NFC/NDF (0.75 and 1.02) and allicin supplementation (0 and 0.04% on a DM basis), resulting in four treatment groups with 10 replicates per group. Increasing dietary NFC/NDF decreased serum pH and increased the anion gap (p < 0.05). An interaction between NFC/NDF and allicin supplementation was observed for the serum concentrations of non-ionized calcium and total calcium (p < 0.05), with the highest concentrations detected in cattle fed high-NFC/NDF diets supplemented with allicin. Serum uric acid was elevated by high NFC/NDF (p < 0.05). High dietary NFC/NDF also increased urine pH (p < 0.05). Significant interactions between dietary NFC/NDF and allicin supplementation were observed for allantoin, creatinine, and urease activity (p < 0.05). High NFC/NDF increased N excretion, total N excretion, and N excretion rate (p < 0.01). Additionally, allicin supplementation reduced fecal N excretion and N excretion rate and increased the apparent total-tract digestibility of N (p < 0.05). Fecal microbiota Alpha diversity analysis showed that high NFC/NDF decreased the Chao1, observed features, and Shannon (p < 0.05). At the phylum level, high NFC/NDF increased Pseudomonadota but decreased Cyanobacteriota, and a significant interaction between NFC/NDF and allicin was observed for Spirochaetota (p < 0.05). At the genus level, high NFC/NDF reduced unclassified_UCG-010 but increased Succinivibrio, unclassified_Muribaculaceae, and Methanobrevibacter (p < 0.05). Allicin increased Rikenellaceae_RC9_gut_group abundance (p < 0.05). In conclusion, dietary NFC/NDF and allicin supplementation modulate acid–base balance, N utilization, and fecal microbial composition in beef bulls, providing a scientific basis for improving feed efficiency.