The dynamic model of post-weaning growth and composition developed by Oltjen et al. (1986) was reparameterized using a data-set of seven experiments with a total of 119 Nellore bulls where dry matter intake (DMI), metabolizable energy (ME) concentrations of the diets, and initial and final empty body (EB) compositions were available. Running the model with the initial parameters (for British breed bulls) resulted in a slight underprediction of final EB protein mass but more serious errors (underpredictions) in body fat and energy, due to higher efficiency of energy utilization by the Nellore animals. An optimization routine was developed to enable parameter estimation, and the entire data-set was used to fit the model to the observed growth of bodycomponents. Fitting the protein deposition of the animals was possible either by: (i) decreased target DNA mass at maturity (DNAmax) and simultaneously increased DNA accretion rate constant (k_1) and reduced protein degradation (DEG) rate constant (k_3);or (ii) fixing DNAmax and fitting k_1 and k_3. The maintenance energy coefficient (a) was fitted as well. The observed protein accretion curves could be due to higher rates of protein synthesis (SYN) or lower rates of protein DEG. The decreased maintenance requirement and data on post-mortem muscle metabolism in Bos indicus animals suggest that decreased DEG is the more likely hypothesis. Biologically, increased DNAmax would imply a larger frame size in Nellore compared to British cattle, but this wasnot supported by the data or the literature, as Nellore animals reach maturity at a similar live weight (LW). Therefore, only k_1, k_3 and a were allowed to change. Sensitivity analyses showed high non-linear correlations between these parameters, thusmore detailed longitudinal data would be required to determine unique solutions. Best fit parameter estimates and insights from the model behaviour against field observations indicate that Nellore animals may have lower rates of DNA accretion (k_1), confirming Nellore's reputation as a slower-maturing breed. This was accompanied by lower rates of protein DEG (k_3), and endogenous energy utilization (alpha). Therefore, these results indicate that an existing model of cattle growth, developed using data from Bos taurus breeds, is capable of simulating the growth and composition of Nellore cattle, as long as the parameters are adjusted accordingly. Parameter adjustments indicate that in comparison with their European counterparts, Nellore cattle are slower-maturing, have lower rates of protein turnover and lower endogenous energy expenditures.
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