Abstract

The objectives of the study were to compare the goodness-of-fit of seven empirical models (orthogonal Legendre polynomials Order 1 to 4; Wood, Ali and Schaeffer and Wilmink functions) to fit lactation curves of three local colour phenotypes (brown, white, and black) of the Chiapas sheep breed; and to evaluate the effect of environmental factors affecting the parameters of the lactation curve of the model of best fit. A total of 428 complete and incomplete lactations from 1 to 5 parities from 339 ewes (151 white, 95 brown, and 182 black) collected in two periods of four consecutive years from 1991 to 1994 and 2005 to 2008 at the Centre for Technology Transfer-Autonomous University of Chiapas, Mexico, were used in the analysis. Estimates of the parameters for each of the models were obtained using random regression mixed models and several criteria for goodness-of-fit were used to compare model fit. Of the models compared, the orthogonal Legendre polynomial of Order 3 had the best fit. Estimates of parameter α₂ were significantly different between sheep colour phenotypes (P < 0.05). The estimate of parameter α₀ was affected only by parity number (P < 0.05). Genetic evaluation for milk yield and shape of the lactation curve can be implemented.

Key words: Chiapas sheep breed, lactation curve, longitudinal models, random regression.