ABSTRACT
Summary Assessing forage quality depends not only upon the characteristics of the plant and animal, but also upon the dynamic interaction of the forage, rumen microorganisms, and animal. Although static variables, such as composition or 48-hour in-vitro fermentation, are important for characterizing feeds and predicting in-vivo responses under standard conditions, they provide little information that can be used to assess forage quality in dynamic situations. Dynamic properties, such as rates of digestion 506and passage, can be used to predict digestion under varied circumstances but require an appropriate mathematical model. Our objectives were to describe a dynamic model of fiber digestion and passage in the ruminant and to illustrate its use in assessing forage quality. In the model, neutral detergent fiber is divided into three fractions: fast-digesting (>0.02/hr), slow-digesting (<0.02/hr), and indigestible. While the indigestible fraction can escape the digestive tract by passage only, the digestible fractions can disappear by both digestion and passage. Forage particles enter the rumen into large-, medium-, or small-particle pools. As particle size is reduced, particles move from large to medium and from medium to small pools. Escape from the rumen can occur only from the small- and medium-particle pools. All rates of passage and digestion are assumed to be first order. The model, which requires 20 differential equations, has been implemented on the computer, using both CSMP and GASP IV simulation languages. The model can be used not only to predict forage quality under varying situations of plant and animal characteristics but also to measure the relative importance of various factors in determining forage quality. For example, the dynamic model predicts that the indigestible fraction has the greatest influence on both digestibility and intake. Our simulations indicate that rate of passage is also an important variable influencing forage intake and digestibility. The dynamic model adds a new dimension for assessing forage quality because it provides flexibility in determining the effects of changes in both the plant and the animal on forage utilization.
