Abstract
The synchronization between nitrogen sources and carbohydrate fractions represents a critical factor for optimizing microbial protein synthesis and overall ruminant performance. This systematic review, conducted according to PRISMA 2020 guidelines, comprehensively evaluated the interactions between different nitrogen sources (true protein, urea, controlled-release urea, and bypass amino acids) and carbohydrate fractions (rapidly degrading soluble, slowly degrading soluble, fibrous, non-fibrous, and Van Soest fractions) in ruminant nutrition. A comprehensive search across PubMed, ScienceDirect, Web of Science, and Scopus databases identified 1855 records, of which 164 studies met the eligibility criteria for qualitative synthesis and 89 for quantitative meta-analysis. The review reveals that synchronization effectiveness varies significantly depending on the nitrogen source-carbohydrate combination, with controlled-release urea showing superior synchrony with slowly degrading carbohydrates, while conventional urea performs better with rapidly degrading sources. Meta-analytical results indicate that optimal nitrogen-carbohydrate synchronization can improve microbial protein synthesis by 18-34%, reduce urinary nitrogen excretion by 12-28%, and enhance feed efficiency by 8-15%. These findings provide evidence-based recommendations for precision nutrition strategies in ruminant production systems.