Evaluation of Different Mixed Model Nonlinear Functions to Describe the Body Weight Growth of Pigs of Different Sire and Dam Lines1

The growth rates of 1,932 barrows and gilts from 3 sire lines and 2 dam lines were evaluated over 2 replicates. The pigs were weighed at birth, weaning, 55 d of age, and approximately 28-d intervals from 74 to 158 d of age. Mixed models were evaluated for the Bridges, Gompertz, 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 3 functions provided similar likelihood statistics, residual SD, predicted BW, and predicted ages to achieve 105 kg BW. The predicted ages and BW at maximal growth were greater and more variable (P > 0.01) for the Bridges and GMM functions than the Gompertz function. The GMM predicted greater mature BW (379.3 kg) than the Bridges (238.7) and Gompertz (211.9) functions. Age at maximal growth, BW at maximal growth, and days to achieve 105 kg BW were affected (P < 0.001) by sire line and sex. Sire line × replicate and sex × replicate interactions (P < 0.05) for the growth function parameters, age at maximal ADG, BW at maximal ADG, and days to achieve 105 kg BW indicated genetic × environmental interactions.