ASAS Early Career Achievement Award Presentation: Advances in modeling ruminant nutrient utilization

Nutrition modeling has been applied to study various aspects of the ruminant such as rumen function, post-absorptive metabolism and product composition. However, the objective of this study will be limited to advances in modeling rumen function with respect to environmental issues and animal health, particularly, rumen disorders. Interest in environmental issues comes from the potential to manipulate rumen functions to reduce energy losses as methane and utilization of nitrogen (N) by rumen microorganisms. Enteric methane production arises principally from microbial fermentation of hydrolysed dietary carbohydrates. A widely used empirical model estimates that 5.5% to 7.5% of gross energy (GE) intake is lost as methane. However, mechanistic models of rumen function are based on the laws of mass and energy conservation and take into account the rumen economy of primary substrates for ruminal methanogenesis (hydrogen and carbon dioxide). Mechanistic models have suggested that 3.8 to 7.4% of GE is lost as methane depending on the diet. They help understand the system under study and therefore provide more insight on how dietary manipulation reduces methane loss than empirical models. One such mitigating option is the use of starch, which, if used excessively, can lead to health problems such as sub-acute rumen acidosis (SARA) due to rumen pH depression. Mathematical models have been developed to describe changes in rumen pH and rumen fermentation. The models were used to determine the maximum amount of nonfiber carbohydrate to be included in the diet above which SARA would likely occur. Models that relate rumen temperature to rumen pH have also been developed and have the potential to aid in the diagnosis of SARA. Several empirical and mechanistic models have been constructed to predict N utilization in the rumen and the form in which N is excreted from the animal. These models show that the magnitude of feces to urine N ratio has implications on ammonia volatilization, nitrate leaching and contribution to greenhouse gas emission as nitrous oxide. A rumen model that integrates nutrient utilization and health has great potential benefit for ruminant nutrition research.