Simulation of growth in pigs: Approximation of protein turn-over parameters

A dynamic model for simulation of growth in pigs was extended by a module to describe protein turn-over in six body protein pools (muscle, connective tissue, liver, blood plasma, gastro-intestinal, and 'other' proteins). The model describes protein deposition in these pools following different growth curves and differential rates of turnover. Growth curve parameters and turn-over rates were obtained from the literature. In growing animals, experimentally measured turn-over rates represent a combination of turn-oner of already-present body protein and fractional (repeated) synthesis of newly deposited protein. An attempt was made to distinguish between these processes by varying the values of the fractional rate of synthesis of newly deposited protein (FRS(dep)) and of the proportion of maintenance energy requirements not related to protein turn-over (FrcME(maint)) and comparing the simulated output to the output from the original model without the protein turn over module. The turn-over rate (TR(pres)) of already present connective tissue protein reached unrealistic values for FXS(dep) >2.5 per day, which puts an upper limit on FRS(dep). The output from the extended and the original models showed similar patterns for certain combinations of FRS(dep) and FrcME(maint), dependent on the levels of model input variables. For FRS(dep)less than or equal to 2.5 per day, these similar patterns have their optimum at FrcME(maint)=0.65, coinciding with FRS(dep)=2.0 per day. The corresponding TR(pres) values were 0.060, 0.019, 0.585, 1.492, 0.582, and 0.016 per day for the above mentioned pools.