Redefining feed efficiency through the livestock gut microbiome.

Feed efficiency remains a central goal in livestock production because it determines both economic viability and environmental performance. Yet conventional measures such as feed conversion ratio and residual feed intake often treat efficiency as a host-level outcome and do not fully capture the biological processes that govern nutrient transformation and use. This perspective argues that the gastrointestinal microbiome is a critical, and still underappreciated, mediator of feed efficiency across livestock systems. In ruminants, rumen microbial communities drive the conversion of fibrous feeds into volatile fatty acids and microbial protein, thereby shaping host energy supply, nitrogen utilization, and methane loss. In monogastrics, intestinal microbiota influence nutrient salvage, short-chain fatty acid production, barrier integrity, immune tone, and metabolic signaling, with direct consequences for growth and productive performance. We contend that feed efficiency should be reframed as an emergent property of diet-microbiome-host interactions rather than as a simple input-output trait. From this viewpoint, microbial mediation helps explain between-animal variation in nutrient bioavailability, digestive stability, inflammatory burden, and resilience under commercial production conditions. We further highlight how microbiome-informed feeding strategies, including dietary bioactives, probiotics, prebiotics, enzymes, and precision nutrition approaches, could improve nutrient conversion while reducing methane emissions and reliance on antibiotics. Recognizing the microbiome as a functional regulator of feed efficiency offers a more mechanistic and sustainability-oriented framework for livestock nutrition research and practice, with important implications for breeding, management, and future multi-omics innovation.