An update of a dynamic model of ruminant digestion

The progressive development of a dynamic model of ruminant digestion for evaluation of factors affecting nutritive value of feedstuffs is outlined. Significant changes have been introduced in many of the interactive subunits that accommodate the digestion of 12 chemical constituents (soluble carbohydrate, organic acids, starch, pectin, hemicellulose, cellulose, lipids, soluble protein, insoluble protein, nonprotein N, lignin and ash), microbial growth, animal interactions and summary computations. Explicit consideration of soluble carbohydrates, organic acids and pectins has been replaced by an aggregate fraction. A section has been added to follow dynamic aspects of volatile fatty acid metabolism in the rumen. Evaluations of the model were made by comparing its behavior with that observed experimentally for sheep fed a number of diets. These included an alfalfa diet fed at two intake levels either hourly or once daily, a dried subterranean clover diet fed hourly, a forage oat diet fed every 3 h and an 80% concentrate diet fed twice daily. For frequently fed diets, model estimates, in general, agreed very well with experimental estimates. Although changes in representations of protein metabolism were helpful, the pool size of ruminal ammonia N was not simulated well. Digestion and passage in the model occurred more rapidly than was observed when animals were fed high-quality forages once daily and rates of particle size reduction, fermentation and passage were lower than observed when low-quality diets were fed. It was concluded that the model can serve as a useful and adaptable tool for analyzing factors affecting nutritive value. Particle size reduction and passage from the rumen, dynamics of protein metabolism and utilization, and water dynamics were identified as areas requiring further research.