Comparative study of prolificacy of Iran black sheep breed with Arman and Baluchi breeds and investigating genetic relationship of prolificacy with average lambs weight per lambing of Iran Black

Prolificacy trait of Iran black was compared with Baluchi and Arman sheep breeds and the environmental factors affecting the trait were studied. Furthermore, estimation of genetic parameters and evaluation of the genetic trend of the Iran black breed for the prolificacy and average birth weight of the lambs per lambing (as a trait for the ewes) were carried out using bivariate (linear-threshold) repeatability model. Total recorded animals was 2,701 (offspring of 110 sire and 840 dam) and the data were collected during 1980-2009 by Abbas Abad breeding station that located in Mashhad, Iran. Lambing rate was significantly higher in Iran black and Arman breeds than in Baluchi breed (P<0.01). Lambing rate decreased with aging of the ewes, such that, ewes older than 8 years old showed the lowest prolificacy. Ewes with age between 2 to 6 years did not differ to each other significantly. Heritability of average birth weight of the lambs and prolificacy were 0.148 and 0.266, respectively. Repeatability estimates of the traits were 0.674 and 0.381, respectively. Phenotypic, environmental and genetic correlations among the traits were highly negative and were -0.484, -0.369 and -0.968, respectively. The genetic trend for the prolificacy trait was ascending until year 1996 (0.016 lamb/year) and descended afterwards (-0.012 lamb/year). Genetic trend for average birth weight of the lambs were opposite in direction with genetic trend of prolificacy in both periods. It seems that lack of a sustainable breeding strategy for genetic improvement of Iran Black breed for prolificacy and other economically important traits has resulted in a non-constant genetic progress during the past years.