Physiological adaptations of ruminants and their potential relevance for production systems

It is easy to propose that ruminants represent the most advanced solution to the typical dilemma that food intake is typically negatively correlated to factors determining digestive efficiency, such as thoroughness of ingesta comminution by chewing, and retention of digesta in the digestive tract. Most ruminants share the characteristic of 'digesta washing': fluid moves through their digestive tract faster than particles, thus effectively washing very fine particles such as bacteria out of the digesta plug. As the forestomach is followed by aut-enzymatic digestion, this allows a continuous, increased harvest of microbes from the forestomach. True rumination only evolved twice, in the camelids and the true ruminants. These both evolved a density-dependent sorting mechanism based on physical separation of digesta by the process of flotation and sedimentation, ensuring that the process of rumination is applied to the larger particles. Differences in this sorting mechanism might facilitate a faster digesta processing in true ruminants as compared to camelids. The hallmark of ruminant digestive anatomy is the omasum, in which the fluid required for both the 'digesta washing' and the reticular separation mechanism is re-absorbed. In cattle-related ruminants the omasum has reached the largest size to date, and this group has a particularly high forestomach fluid throughput. Increasing the degree of digesta washing even more should increase microbial harvest from the forestomach and reduce the susceptibility to acidosis. At the same time, it should result in a metabolic state of the microbiome more tuned towards biomass production and less towards methanogenesis. Enhancing forestomach fluid throughput by selective breeding could represent a promising avenue to further advance the productivity of the ruminant digestive tract.