Evaluation of a growth model of preruminant calves and modifications to simulate shortterm responses to changes in protein intake

Recently, a mechanistic simulation model was developed which integrates protein and energy metabolism of preruminant calves of 80 to 240 kg live-weight (LW). The model can be used to predict the rate of gain of weight, protein and fat from nutrient input. A serial slaughter experiment was conducted to obtain an independent data set for model evaluation purposes. The objective of the model evaluation exercise was to evaluate model predictions to different protein intakes in three LW ranges: 80-120 kg (three levels of protein intake), 120-160 kg (three levels of protein intake) and 160-240 kg LW (two levels of protein intake). In this experiment, 64 calves were used, of which 36 were slaughtered to determine chemical body composition. Four calves were slaughtered as a reference group at 80 kg LW and subsequently four per protein intake level at each of the pre-determined target weights. Model predictions were found satisfactory for live-weight gain and protein deposition, with root mean-square prediction errors (RMSPE) expressed as a percentage of the observed mean of below 7%. For fat deposition, the RMSPE was 18% (on average 26 g day-1), of which 58% was due to a consistent underestimation. Closer examination of the experimental results from 80-120 kg LW revealed an adaptation of the calves to the sudden change in diets after the start of the experiment, still present at 120 kg LW and not predicted by the model. Modifications were made to the model, based on the hypotheses that (i) calves adapt their visceral protein mass quickly to a new feeding level, and (ii) that the capacity of calves to make these corrections decreases with age or live-weight. The adapted model satisfactorily predicted shortterm responses in live weight gain to sudden changes in protein intake but remains to be rigorously evaluated. The approach taken may also be valid for modelling shortterm responses in intakes of other nutrients. It can, however, not be used when animals can adapt their rate of intake in an attempt to compensate for a nutrient deficiency (e.g. during qualitative feed restriction).