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چکیده
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The objectives of our study were to compare different non-linear mixed effect models (Brody, Von Bertalanffy, Richards, Gompertz and Logistic) for best description the growth patterns of Markhoz kids, evaluate the influence of environmental (fixed) effects on growth curve parameters of the best-fit growth model and finally estimate the genetic parameters for growth curve characteristics. Live weights of 3678 individuals from the historical archives of the Markhoz Goat Performance Testing Station (in Sanandaj), which were collected from birth to 365 days of age during 1992 to 2015, were used in the analysis. Models were fitted according to the stochastic approximation expectation-maximization (SAEM) procedure implemented in R package SAEMIX. All studied mixed models described the growth pattern in Markhoz goat, except Richards model. However, the Brody model best fit the data. The analysis of variance on the growth curve parameters (at the individual level) showed that among the fixed effects (i.e., sex, birth type, age of dam and year of birth) for the Brody model, only age of dam had no effect on any of the growth parameters. Estimates of heritability for growth curve parameters were low, with 0.067 ± 0.011, 0.063 ± 0.010 and 0.044 ± 0.001 for mature weight (a), constant of integration (b) and rate of maturing (k), respectively. Low heritability estimates suggest that the environment is not favorable for expression of genetic potential of Markhoz kids. The negative genetic (−0.866 ± 0.000) and phenotypic (−0.723 ± 0.008) correlations between the parameters a and k indicates that animals with lower growth rates are more likely to reach higher weight values when adults.
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