عنوان مقاله [English]
نویسنده [English]چکیده [English]
This study was carried out to investigate the effect of selection on the reduction of fat-tail size in Lori-Bakhtiari lambs. The study lasted for five years at the Development and Breeding center of Lori-Bakhtiari Sheep Station. Estimation of fat-tail weight, using external fat-tail dimensions was obtained from regression procedure at 6 month age. Soft tissue depth at the point 12 cm from the midline dorsal body on 12th ribs at the age of six months (using an animal model of Scanner Device 480) was determined. Components of (co) variance and genetic parameters were estimated using multivariate animal model and a restricted maximum likelihood approach. Selection as based on economic selection index of multiplying a vector of breeding value traits by vector of economic values of traits was conducted. Coefficients of 1 and -4 for the economic value of body weight and estimation of fat-tail weight traits were applied. Phenotypic and genetic trends were obtained through regression means of phenotype while breeding values on year of birth for each trait being predicted. The results showed that average of fat-tail weight (estimated at the age of six months) in lambs amounted to 2.37 kg. The estimates of heritability and standard error fat-tail amounted to 0.33 ± 0.05. Genetic trend of body weight at the six months of age was 160 gr. The genetic trend of fat-tail weight was estimated as -40 gr, with a high coefficient of determination of 0.94. Depth of soft tissue from ultrasound and at six months of age was accompanied by a negative genetic trend (-0.024 mm). Change of phenotype of body weight and ultrasound soft tissue depths were estimated as positive and more than genetic variations. However, phenotypic trend reduction of fat-tail weight as compared with the genetic trend stood at a lower level.