DEVELOPMENT OF A MECHANISTIC MODEL OF INTAKE AND CHEWING ACTIVITIES OF SHEEP

A mechanistic model of intake and chewing activities was developed usi ng data from confined sheep in order to integrate the relationships be tween feeding behavior and digestive processes. The model consists of two interconnected submodels. The ruminal digestion submodel describes flows of nutrients and is based on differential equations to simulate the dynamic evolution of particulate matter and volatile fatty acids (VFA) in the reticulorumen. The diet is characterized by cell wall con tent and its potential digestibility, by the proportion of large parti cles (LP) retained on a l-mm mesh sieve, and by an index of palatabili ty. Particle comminution occurs during eating and ruminating. Intake i s determined from attributes of the diet, animal live weight, and sati ety status. Particulate outflow is calculated from a description of th e activity of the reticulo-omasal orifice. Microbial digestion rates v ary with lag phase, chemical fraction, size of particles, and ruminal pH. The VFA are aggregated into one compartment. The feeding decision submodel distinguishes among eating, ruminating, and resting. The choi ce among these activities is decided at each minute of simulation acco rding to the relative values of functions of intake motivation (FMI) a nd of satiety (FSAT). The FMI function is based on diet palatability, energy balance, and the diurnal cycle. The FSAT function is determined by rumen load signals and energy balance. When the animal does not ea t, the decision between ruminating and resting is related to the propo rtion of long particles in the rumen. Sensitivity analysis and validat ions indicate that the overall behavior of the model is adequate.