Estimation of genetic, phenotypic and environmental trends for body weights at different ages in Lori sheep

Introduction Lori sheep is one of the most important breeds in Iran that is mostly bred in Lorestan province, north-east of Khuzestan and some areas of Ilam provinces. The name of this breed is derived from Lori tribe, which is one of the major nomadic groups of Zagros Mountain in western areas of Iran. Lori sheep is resistant to mountain conditions and hot and dry plains. This sheep breed has strong constitution, good traveling ability with suitable conformation as a mountain sheep. It is one of the predisposed breeds to fattening and often is traditionally kept by villagers and nomadic tribes in the area. In such a system output is lower than in an intensive system. Meat production in Iran is one of the most valuable traits for livestock breeders. Also, sheep meat has remarkable values rather than the meat of other animals and is popular between people. Thus accurate estimations of genetic parameters of these traits are considered by breeders. Accurate prediction of breeding value of animals is one of the best tools available to maximize genetic gain. Success of a breeding programme can be evaluated by actual change in breeding value expressed as a proportion of expected theoretical change of the breeding value mean for the trait under selection. Several methods are being developed to measuring the genetic changes in the animal population. Carrying out experiments in comparable environmental conditions over a period of several generations is difficult, thus genetic trend estimation is problematic over time and changes in performance may reflect, to some extent, both environmental and genetic changes. In order to overcome such a problem, Hill (1972) proposed that by simultaneously maintaining a control population it is possible to removing the effect of environmental changes, but this is not cost-effective, particularly over a long period of time. Best linear unbiased prediction (BLUP) is the best approach for prediction of breeding values and estimation genetic gain. Little information is available on the estimation genetic trends for body weight traits in Lori sheep. Hereupon the objective of the present study was to estimate genetic, phenotypic and environmental trends for body weight traits at different ages in Lori sheep to assess the breeding programmes.
Material and Methods In the current study, 6440, 5646, 5073 and 4757 records of birth weight (BW), weaning weigh (WW), 6-month weigh (6MW) and 9-month weigh (9MW) of Lori sheep were used. Data were collected from 2001 to 2010 by the agricultural and natural resources research centre of Lorestan province Lorestan Agricultural & Natural Resources Research Center. Direct and maternal heritability were estimated using restricted maximum likelihood by fitting six uni-variate animal models. Akaike’s information criterion (AIC) was used to selecting (to select or in/for selecting) the most appropriate model for each trait. Breeding values were estimated by the best model for uni-variate analysis. Genetic, phenotypic and environmental trends were calculated by regression of means of predicted breeding values, phenotypic means and difference between estimated means of breeding values and phenotypic means on birth year for each trait, respectively. Wombat software was performed to obtaining direct and maternal (co) variance components and heritability estimations for body weight at different ages in Lori sheep. Genetic trends analyses were performed by the regression procedure of the SAS software package.
Results and Discussion The phenotypic trends for BW, WW, 6MW and 9MW were 0.016, 0.065, 0.032 and -0.783 kg per year, respectively. The direct genetic trends for BW, WW, 6MW and 9MW were 0.008, 0.001, 0.055 and 0.076 kg per year, respectively. The direct genetic trends were significant for all of the traits with the exception of 9MW. Genetic gain for BW, WW, 6MW and 9MW were 0.061, 0.007, 0.139 and 0.150 kg after 10 years, respectively. Hence, genetic gain for all of the studied traits was low.
Conclusion In studied flock, the effective major factors in low genetic gain were the lack of a proper selection criteria and environmental and management conditions changes. Agricultural Jihad Organisation of Lorestan province has performed a breeding programme to improvinge Lori sheep by replacing the original breed rams in local flocks. This approach is very suitable and it is necessary to continue. Economic selection index method is an important tool that can help to improving (improving/to improve) genetic progress for body weight traits at Lori sheep.