Estimation of Inbreeding and Its Effects on Growth Traits in Sangsari Sheeps

The aim of this study was to estimate inbreeding coefficient in Sangsari sheep and its impact on growth traits in order to manage breeding programs and preventing possible negative and harmful effects of excessive increase of inbreeding. Inbreeding is the mating of individuals whose relatedness is greater than the average degree of relationship exists in the population and capable of changing the genotypic frequencies of a population without modifying the gene frequencies. Increase in inbreeding can lead to reduced response to selection of economic traits and the heterozygosity and consequently leads to increase in homozygosity and loss of genetic diversity. Inbreeding also impairs growth, production, health, reproduction and survival of inbred animals. A complete pedigree is also necessary for accurate evaluation of inbreeding and other important population parameters.
In the present study, data from 7028 Sangsari sheep (3180 males and 3848 females) which were collected during 28years (1987-2014) in the breeding station of Sangsari sheep located near to Damghan city, Semnan province, were used to estimate inbreeding coefficients and its effects on birth weight (BW), weaning weight (WW), 6 month weight (6MW), 9 month weight (9MW) and yearling weight (YW) of animals. Pedigree information for each lamb included animal, sire, and dam and also sex, birth date, type of birth, age of dam and record of studied traits. In order to edit data, FoxPro software (FoxPro 2.6) and to study of fixed effects on growth traits, GLM procedure of SAS 9.1 were used. Before estimation of inbreeding, it is very important to be sure about completeness and depth of the pedigree. In order to estimate inbreeding coefficients of all animals in the pedigree and to determine the population structure, CFC based on Meuwissen and Luo algorithm and Endog v4.8 softwares, were used. The regression of the studied traits on inbreeding coefficient was estimated using WOMBAT software by restricted maximum likelihood method and 12 animal models were used. The most appropriate model for each trait was determined based on Akaike’s information criterion.
Of all the registered animals, 4772 animals had both parent known. Completeness of the first and second ancestor generations of all animals were 78.52% and 44.33%, respectively, and the completeness decreased progressively for the next generations. The average equivalent complete generation, as a measure of pedigree completeness level, estimated as 1.59. In the studied pedigree, A low proportion of the recorded lambs (7.21percent of the total lambs) were inbred and their average inbreeding coefficients was 3.88 percent. Mean inbreeding of the total population and average relatedness estimated as 0.28% and 0.59%, respectively. In the early years, the number of inbred animals was low, but over time, the average coefficient of inbreeding increased due to the selection of breeding male and female animals within the flock and mating of the related animals and inbreeding coefficient was increased by 0.028 percent per year and was statistically significant. Maximum and minimum inbreeding coefficients were 31.25 percent and 0.024 percent, respectively, in inbred population. Low average coefficient of inbreeding can be mostly due to controlled mating in the flock and also due to having many animals with inbreeding coefficients of zero. Zero inbreeding coefficients were mainly due to lack of pedigree information in the early years. Effects of inbreeding on growth traits were estimated by restricted maximum likelihood method using 12 animal models. The most appropriate model for each trait was determined based on Akaike’s Information Criterion. The regression coefficients of inbreeding on birth weight (model 11), weaning weight (model 8), 6 month weight (model 11), 9 month weight (model 9) and yearling weight (model 3) were -1.46, -7.72, 14.87, 20.92 and -29.46 gram, respectively, which were not statistically significant.
Generally, results indicated that the average coefficient of inbreeding in the studied Sangsari sheep population was lower than the values reported in other breeds which could be because of lack of clear information of some parents and common ancestors and also because of the purposive and largely controlled matings therefore there is no serious problem of inbreeding and its effects on body weights in this population, but in order to prohibit the animals with high inbreeding levels, pedigree information has to be considered during mating.