Comparison of fixed effects and mixed model growth functions in modelling and predicting live weight in pigs

The objective of the present study was to compare alternative growth functions fitted to pig live weight data, with particular emphasis on the function׳s ability to predict future animal live weight. The final dataset consisted of 51,893 live weight records from 10,201 pigs aged between 61 and 200 days. Fixed effects models and mixed models were applied to the three different growth functions: von Bertalanffy, Gompertz, and Richards; fixed and mixed model polynomial equations were also considered. The growth function׳s ability to predict future live weight was determined by excluding a selection of animal live weight records post 160 days of age and comparing the predicted live weight to actual live weight. Irrespective of whether a fixed or mixed model framework was used, the Gompertz function best modelled the full dataset, with an accuracy squared for the full dataset (rw2) of 0.997 and root mean square error (RMSE) of 1.60 kg for the fixed effects model and rw2 of 0.995 and RMSE of 2.25 kg for the mixed model equation. The Gompertz mixed model also achieved the greatest accuracy squared of predicting (rww^2) future live weight records, with an rww^2 of 0.846 and RMSE of 5.35 kg. A strong positive correlation was estimated between asymptotic mature weight (A; r=0.83 to 0.96) across the three growth functions for all considered equations. A strong negative correlation was estimated between parameters A and k (rate of maturation) for the Gompertz fixed effects model (−0.88) and mixed model (−0.70). Results from this study could be used to implement a decision support tool for pork producers, offering extra information when they are making important feeding, slaughter and breeding decisions.