estimation of genetic parameters for average daily gains and kleiber ratios in guilan province sheep

introduction: the genetic characterization of local breeds of sheep is of paramount importance, not only for conservation purposes but also for the definition of breeding objectives and breeding programmes. when records are nonexistent and breeds are not well defined in the field, information could be obtained from research station information where pedigree and performance records have been kept. early growth of animals is mostly dependent on the animal’s genetic merit for growth and maternal genetic and permanent environmental effects. hence, both the direct and maternal genetic effects should be taken into account in a selection program to achieve optimum genetic progress. studies on different sheep breeds have indicated that both direct and maternal genetic effects are essential for lamb growth. the efficiency of any sheep production enterprises can be improved through considering economically important traits such as litter size, body weight of lamb and quantity and quality of wool in the breeding objective. growth is an economically important trait of farm animals which can be interpreted mathematically. it is a time (age) dependent change expressed in the weight or size of organ, composition of tissue/organ, size or number of cells and in live weight. growth rate is related to rate of maturing and mature weight and these traits have been suggested to have association with lifetime productivity parameters in animals. slow growth rate causes low market weight and has been identified as one of the limiting factors affecting the profitability of any production system. selection of animals based on their predicted breeding values and accurate estimation of genetic parameters for economically important traits are required for designing optimal breeding strategies for farm animals. the kleiber ratio has been suggested to be a useful indicator of growth efficiency and an indirect selection criterion for feed conversion under field conditions. the guilan sheep is a fat-tailed breed of domestic sheep in iran, numbering some 400,000 animals in the north of the country, and distributed in the northern and western parts of guilan province in the mountains between assalem, khalkhal, oshkourat, and deilaman. this breed can also be found in some areas of guilan-zanjan border. mean adult live weight in this breed is 35 kg (77 lbs) for rams and 31 kg (67 lbs) for ewes. the coat color for this breed is yellowish-white to pure white, but brown patches are found on the head, face and at the bottom of the legs. this breed is valued mainly due to its small size, meat, and ability to live in mountainous areas with rain-fed foothills and foothill steppes with 1300 mm (51 inches) rain. the objective of the present study was to estimate genetic parameters for average daily gain from birth to 3 (adga), 6-month of age (adgb), average daily gain from 3 to 6-month of age (adgc) and corresponding kleiber ratios (kra، krb و krc) in guilan province sheep. materials and methods: data and pedigree information used in this study were collected during years 1994 to 2011 by the agriculture organization of guilan province, iran. the glm procedure of sas was used for determining the fixed effects which had significant influence on the under study traits. level of significance for the inclusion of effects into the model of analysis was declared at p<0.05. flock-year-season of lambing, lamb’s sex and dam’s age at lambing had significant effects on the traits. genetic parameters were estimated with 6 different animal models using restricted maximum likelihood (reml) procedure of wombat program. the most suitable model was determined based on akaike’s information criterion (aic). results and discussion: based on the most appropriate fitted model, direct heritabilities of adga, adgb, adgc and corresponding kleiber ratios were estimated to be 0.79, 0.18, 0.06, 0.25, 0.53 and 0.07, respectively. the lowest estimate of direct genetic correlation was between krb-krc (-0.48) and the highest estimate of direct genetic correlation was between adgc-kra (0.94). estimated phenotypic correlations between average daily gains and kleiber ratios were from -0.37 to 0.90 and those of environmental correlations were from -0.55 to 0.90. estimations of genetic parameters for growth traits in this study showed that there are enough genetic diversity and as much as other breeds of sheep in iran in guilan province sheep and also genetic progress for these traits is possible by selection.