Evaluation of Different Mixed Model Nonlinear Functions on Pigs Fed Low-Nutrient Excretion Diets1

Three nonlinear growth functions were evaluated on barrows (n=108) and gilts (n=105) fed diets designed to reduce nitrogen and phosphorous excretion. The pigs were weighed at birth and at approximate 14-d intervals from 62 to 147 d of age and then weekly to 190 d of age. Mixed models including pig specific random effects were evaluated for the Bridges, Weibull, and generalized Michaelis-Menten (GMM) functions. Two pig specific random effects were included in each function-one for predicted mature BW and one variable associated with the age at which maximum ADG was achieved. Alternative analyses were completed in which a random effect for a third parameter was predicted as a linear function of the random effect for mature BW. The alternative analyses resulted in improved likelihood statistics and smaller standard errors for the between-pig variance in mature BW. The 3 functions provided similar likelihood statistics, residual standard deviations, predicted BW, and predicted mean age (140 d for barrows and 134 d of age for gilts) and mean BW (83 kg for barrows and 73 kg for gilts) at which maximum ADG was achieved. The Weibull and Bridges functions predicted that 3 barrows and 6 gilts had mature BW of less than 130 kg. The GMM function allowed for more gradual growth to 50% greater mature BW than the Weibull and Bridges functions. The GMM equation allows for nonsymmetric sigmoidal growth and in some cases may provide a better fit to the BW data of pigs.