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斦 Model 1
y=Xb斦빦鞩篧 Model 2
y =Xb斦빦榾 Cov(a,m)=0 Model 3
y =Xb斦빦榾 Cov(a,m)=Aσam Model 4
y =Xb斦빦榾暗ꥻ Cov(a,m)=0 Model 5
y =Xb斦빦榾暗ꥻ 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 (PConclusion 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.