SIMULATION OF THE DYNAMICS OF PROTOZOA IN THE RUMEN

A modified mathematical model is described that simulates the dynamics of rumen micro-organisms, with specific emphasis on the rumen protozo a. The model is driven by continuous inputs of nutrients and consists of nineteen state variables, which represent the N, carbohydrate, fatt y acid and microbial pools in the rumen. Several protozoal characteris tics were represented in the model, including preference for utilizati on df starch and sugars compared with fibre, and of insoluble compared with soluble protein; engulfment and storage of starch; no utilizatio n of NH3 to synthesize amino acids; engulfment and digestion of bacter ia and protozoa; selective retention within the rumen; death and lysis related to nutrient availability. Comparisons between model predictio ns and experimental observations showed reasonable agreement for proto zoal biomass in the rumen, but protozoal turnover time was not predict ed well. Sensitivity analyses highlighted the need for more reliable e stimates of bacterial engulfment rate, protozoal maintenance requireme nt, and death rate. Simulated protozoal biomass was increased rapidly in response to increases in dietary starch content, but further increa ses in starch content of a high-concentrate diet caused protozoal mass to decline. Increasing the sugar content of a concentrate diet, decre ased protozoa, while moderate elevations of the sugar content on a rou ghage diet increased protozoal biomass. Simulated protozoal biomass di d not change in response to variations in dietary neutral-detergent fi bre (NDF) content. Reductions in dietary N resulted in an increased pr otozoal biomass. Depending on the basal intake level and dietary compo sition, protozoal concentration in the rumen was either increased or d ecreased by changes in feed intake level. Such changes in relative amo unts of protozoal and bacterial biomass markedly affected the supply o f nutrients available for absorption. The integration of protozoal, ba cterial and dietary characteristics through mathematical representatio n provided an improved understanding of mechanisms of protozoal respon ses to changes in dietary inputs.