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

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AbstractThree 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 (83kg for barrows and 73kg 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 130kg. 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.