Survival analysis from birth to yearling age of Lori-Bakhtiari lambs

Data set used in this study were 6800 records of lamb's length life and survival rate from 263 sires and 1839 dams collected from 1989 through 2009, from the Lori-Bakhtiari flock at Shooli station in Shahrekord. The data were analyzed using linear models and hazard ratio models with Weibul function. These models included fixed factors such as year and month of birth, sex of lamb, type of birth, age of dam and lambs birth weight as quadratic covarite, and direct additive genetic, maternal additive genetic, maternal common environmental and residual random effects. Genetic parameters were estimated using restricted maximum likelihood procedure applying with different animal models contained with and without maternal and common environmental effects and sire model. Results showed that the overall mean of lamb's life length and cumulative survival rate up to yearling were 301.60 days and 78.68%, respectively. The effect of fixed factors on survival rate resulted from different linear models were the same as hazard ratio models with Wiebull function. The effect of fixed factors year and month of birth, sex of lamb and lamb's birth weight as quadratic were significant (p0.05), but the effect of age of dam and type of birth were not significant (p0.05) on survival rate. The direct heritabilities of length of life and survival rate estimated from different linear models were low (0.01 to 0.09). The maternal heritability ranged from zero to 0.04, and decreaced by increasing age of lamb. The estimates of heritability in logarithmic scale, original scale and effective heritability obtained from sire and animal models with Weibull function were medium to high (0.08 to 0.55) and higher than analogous values estimated by different linear models. The maternal heritability estimated with Weibull models decreased by age of lamb (0.25 to 0.01). Thus, survival analysis using linear and Weibull models culminated in similar estimates for fixed factors and nonsimilar estimates for random factors. The survival rate up to yearling in lambs can be improved first by farm management practices and improving environmental factors and basis on the genetic parameters estimated from linear and Weibull models, response to selection within breed will be low and medium, respectively.