Altering the representation of hormones and adding consideration of gestational metabolism in a metabolic cow model reduced prediction errors.

The model of R. L. Baldwin predicts various aspects of digestion and metabolism in the cow including nutrient partitioning between milk and body stores. However, prediction bias has been observed for body weight (BW) and body condition score (BCS) when diets of differing energy density are simulated over long periods. Originally, the model overpredicted BW loss in early lactation and gain in late lactation. This bias was reversed and limited to early lactation when a better representation of milk synthesis capacity was introduced into the model. It was hypothesized that a better representation of the effects of energy status on anabolic and catabolic hormones and a more complete representation of metabolic demands and growth associated with pregnancy would help in improving predictions of body tissue mobilization in early lactation. Providing independent glucose reference points and independent sensitivity scalars for the 3 hormones driven by glucose concentrations improved overall model precision. These improvements were primarily realized through reductions in prediction errors for blood glucose concentrations and BCS. In both cases, slope bias associated with the predictions was reduced, indicating that the changes in representation were beneficial although BCS bias was not completely removed. Milk component yields were predicted with slightly greater mean and slope bias. The addition of enhanced pregnancy calculations did not provide apparent additional benefit relative to model prediction errors. However, the data used for the assessments did not include observations from the last 60 d of gestation, where BW gain and metabolic demand associated with pregnancy would be expected to be greater. Improvements in BCS were not observed when the revised model was tested using an independent data set. Predictions of blood fatty acids, the rate of BCS and BW loss, and milk fat yields in early lactation were still inappropriate and require further work. The results could be caused by inaccurate early lactation intakes, the aggregated representation of blood fatty acids, or an inadequate representation of peripheral insulin resistance during early lactation.