estimation of genetic parameters of kleiber ratio and growth traits in kurdish sheep

introduction kurdish sheep breed is one of the most important native breeds of iran. they are fat-tailed, large-sized, well adapted to the mountainous regions in northern khorasan province and mainly raising for meat production under pastoral production system (28). feed efficiency is a major component in the profitability of the small ruminant enterprise, because quality of range and pasture is low in poor environmental conditions in iran. growth rate and feed efficiency are two traits of great economic importance in sheep production and also kleiber ratio has been suggested to be a useful indicator for these traits (2). there was no information regarding genetic parameters for growth traits in kurdish sheep. thus, the main objective of the present research was to estimate (co)variance components and genetic parameters for pre- and post-weaning growth traits and kleiber ratio in kurdish sheep. material and methods in this study, the records of growth traits from 5144 lambs (from 161 rams and 1982 ewes) were used. the data were collected during a 17-year period (1996–2013) in kurdish sheep breeding station located in shirvan city of northern khorasan province. traits investigated were average daily gain from birth to weaning (adg0-3), average daily gain from weaning to six months of age (adg3-6), average daily gain from six to nine months of age (adg6-9), average daily gain from nine to twelve months of age (adg9-12) and kleiber ratios (kr) defined as: kr1=adg0-3/(bw3)0.75 kr2=adg3-6/(bw6)0.75 kr3=adg6-9/(bw9)0.75 kr4=adg9-12/(bw12)0.75 test of significance for the fixed effects to be included in the final functional model for each trait and calculation of least squares means was accomplished using glm procedure of sas software (24). the considered fixed effects were year of lambing (1996-2013), sex of lamb (male and female), type of birth (single and twin) and age of ewe (1–7 years old). (co) variance components and genetic parameters were estimated applying restricted maximum likelihood (reml) method fitting six animal models using wombat (18): y =xb+zaa+e model 1 y=xb+zaa+zpepe+e model 2 y =xb+zaa+zmm+e cov(a,m)=0 model 3 y =xb+zaa+zmm+e cov(a,m)=aσam model 4 y =xb+zaa+zmm+zpepe+e cov(a,m)=0 model 5 y =xb+zaa+zmm+zpepe+e cov(a,m)= aσam model 6 where y: is a vector of records, b: is a vector of fixed effects, a: is a vector of direct additive genetic effects, m: is a vector of maternal additive genetic effects, pe: is a vector of permanent environmental effects due to ewe, x, za, zm and zpe are corresponding design matrices relating the fixed effects, direct additive genetic effects, maternal additive genetic effects and permanent environmental effects due to ewe to vector of y, respectively, e: is a vector of residual effects, and cov(a,m): is the covariance between direct additive genetic and maternal additive genetic effects. akaike’s information criterion (aic) was used for selecting the best model among the tested models (3): where logli: is the maximized log likelihood of model i at convergence and pi: is the number of the parameters in each model. model with the lowest aic was considered as the best model for each trait. estimation of genetic and phenotypic correlations was accomplished using multi-trait analysis (with model 1). the fixed effects included in the multi-trait animal models were those in single-trait analyses. results and discussion the pre-weaning average daily gain in kurdish lambs was 215.33±0.96 g, while this trait in post-weaning periods had a decreased trend. these values (especially in pre-weaning period) indicated that kurdish lambs have a good potential for growth. results from the analysis of variance of adg and kr in different ages showed that birth year and sex of the lambs significantly influenced studied traits (p< 0.01). type of birth had significant effect (p< 0.01) on adg0-3, adg3-6, kr1 and kr2. the effect of ewe age was significant for adg0-3 and kr1. the significant effects of fixed factors on adg and kr traits can be explained in part by differences in endocrine system of male and female lambs, limited uterine space (especially in young ewes), insufficient availability of nutrients during pregnancy and early lactation and competition for milk consumption between twin lambs. the model including direct additive genetic effects, maternal genetic effects as well as maternal permanent environmental effects, without considering covariance between them (model 5) was determined as the most appropriate model for adg0-3 and kr1. the most appropriate model for adg3-6 and kr2 was included a maternal genetic effects as well as direct additive genetic effects, with considering covariance between them (model 4). direct heritability estimates for the considered traits were relatively medium ranging from 0.11 (adg0-3 and kr1) to 0.19 (kr4). estimated maternal heritability and ratio of maternal permanent environmental effects to phenotypic variance for adg0-3 was 0.11 and 0.04, respectively. estimated values for direct and maternal heritabilities of adg traits were well consistent with some of the published values.direct heritability estimate for kr1 was higher than maternal heritability and lower than ratio of maternal permanent environmental effects to phenotypic variance estimates (0.11, 0.06 and 0.12, respectively). the relatively low heritability estimates for the studied traits can be perhaps explained by the low nutritional management, low quality of pastures and harsh climaticb conditions, which result in a high environmental variance. genetic correlation among adg traits varied from 0.18 (adg3-6-adg9-12) to 0.57 (adg0-3-adg3-6). similar to our estimate, positive correlations between adg traits have been reported by abegaz et al. (2) for horro sheep. absoulate value of phenotypic correlation between adg traits were 0.02 for adg3-6-adg9-12 to 0.23 for adg0-3-adg3-6. positive genetic correlations between adg0-3 and adg3-6 in the presence of negative phenotypic correlations might have arisen as a result of compensatory growth mediated through environmental effects in lambs that were gaining at lower rates during the pre-weaning period. direct genetic correlations among adg and kr traits were positive and medium to high. genetic correlation among adg0-3-kr1, adg3-6-kr2, adg6-9-kr3 and adg9-12-kr4 were 0.98, 0.99, 0.98 and 0.97, respectively. estimates of phenotypic and genetic correlations for adg and kr traits were in consistent with those obtained by abegaz et al. (2). conclusion it was observed that maternal genetic models could better explain the genetic variation observed in pre-weaning traits. estimates of phenotypic and genetic correlations among adg and kr traits were high in magnitude. these results suggest that selection for kleiber ratio can result in genetic improvement of growth rate as well as feed effeciency.