INVITED REVIEW: Applied nutrition of ruminants: Fermentation and digestive physiology1

Since the first observation of the 4 chambers and subsequent identification of its microbial population, the rumen has been investigated for its role in nutrient digestion and to manipulate its microbial ecosystem to increase animal performance and efficiency. Ruminants have the ability to digest plant polysaccharides through substrate-specific enzyme activities of the highly specific population of ruminal bacteria, protozoa, and fungi. A synergistic relationship provides these microorganisms with nutrients, desirable ambient temperature, and a buffered environment to enhance microbial growth, and the microbes provide the host animal with B vitamins, VFA, and microbial protein. Microbial populations are not static and fluctuate with changes in the ruminal environment and diet, resulting in changes in VFA composition and concentration. Recent research using terminal restriction fragment length polymorphism analysis, 16S ribosomal DNA libraries, and metagenomic approaches has increased our understanding of the importance of microbial communities within the rumen and the microbial diversity under different dietary environments. Enhanced understanding of these changes in microbial communities could result in methods to enhance microbial growth rates and enzyme secretions in the rumen, resulting in increased digestibility, nutrients delivered to the small intestine, and improved animal production and efficiency. In addition, increased understanding of molecular-level adaptation of ruminal epithelia and gastrointestinal mucosa could provide the physiological basis for their role in regulation of ruminal pH and nutrient transport. Recent advances and continued research in the area of ruminal fermentation and digestive physiology have the potential to positively affect animal production, health, and the environment.