Evaluation of inbreeding depression in growth and egg production traits of birds population in the breeding station of Khorasan Razavi indigenous fowl

Introduction:
Sustainable use, development and conservation of poultry genetic resources are of particular importance for agriculture, food production, rural development and the environment (Besbes et al. 2007). Indigenous chickens are often considered as a support for low input backyard or free-range system of production in rural, semi-urban and nomadic areas (Haunshi et al. 2019). Native chickens have unique characteristics such as ability to adapt to adverse environments, hardiness, broodiness and aggressiveness to protect their chicks, and more. However, low genetic potential is one of the identified limitations for increasing the productivity of native chickens along with inadequate nutrition and disease prevalence (Magothe et al. 2012a).
The inbreeding accumulation and the decline in genetic diversity that occurs due to the mating of related animals in closed populations negatively affect phenotypic values and additive genetic variance (Falconer and Mackay 1996). Inbreeding, which occurs because of mating of related animals in a population, often occurs in the closed and small populations. The negative consequence of inbreeding is referred to as inbreeding depression, which has become a major concern in research related to evolution, ecology, and conservation biology (Reed et al. 2012). In small populations in particular, the inbreeding depression is significant in reproduction issues and has important consequences for the population's ability to evolve and adapt to environmental changes and ultimately their longevity (Leroy 2014).
Increasing the inbreeding rate in closed populations of indigenous chickens' farms may negatively affect the increase in population productivity following the selection process and have destructive effects on improving economic traits in the population. Therefore, considering the importance of the issue, it seems necessary to monitor the inbreeding rate as well as the resulting depression in the under selection closed population of breeding station of Khorasan Razavi indigenous fowl.
Materials and methods:
In order to estimate the inbreeding rate and evaluate its effects on some important economic traits the pedigree data of 38489 birds collected during 2006-2017 in breeding station of Khorasan Razavi indigenous fowl were used. First, the pedigree data of the studied traits were edited using Foxpro and Excell software and then the files required for data analysis were prepared. The eight traits including average egg weight (AEW), egg number (EN), egg weight at 1st day of laying (EW1), age at sexual maturity (ASM), weight at sexual maturity (WSM), body weight at 12 weeks of age (BW12), body weight at hatch (BW1) and body weight at 8 weeks of age (BW8) were studied. The inbreeding coefficients of all birds recorded in the pedigree were estimated using the CFC program (Sergolzai et al. 2006). The inbreeding coefficients of the birds were divided into different categories. Then, the mean changes of inbreeding during different generations were evaluated and the amount of inbreeding changes in each year was calculated using SPSS software by fitting linear regression of inbreeding per generation. Also in this study, regression coefficients of studied traits on individual and maternal inbreeding percentage were estimated with the REML procedure of WOMBAT software (Meyer 2007) through different models.

Results and discussion:
The results showed that about 57% (21892 birds) of all birds registered in the pedigree were inbred. The mean of individual and maternal inbreeding varied relatively little over 12 generations. The mean inbreeding of all birds and inbred birds was estimated to be two and three percent, respectively. The highest and lowest inbreeding rates were three and 0%, respectively. As the results show, despite the closed raising and selection of the population and also the increase in the probability of inbreeding in the population, the inbreeding rate has been increased with a relatively low slope over the generations. In general, during the studied generations, the average coefficient of individual and maternal inbreeding in the whole population showed an increasing trend, so that by fitting the linear regression of inbreeding to the generation, the rate of changes in individual and maternal inbreeding was estimated to be about 0.005± 0.03 and 0.004± 0.02 per generation, respectively. In a study conducted on the Isfahan improved native chicken population (Jelokhani et al. 1400), researchers estimated the rate of individual and maternal inbreeding changes per year for the all population by fitting linear regression of inbreeding per generation to 0.006± 0.05 and 0.005± 0.04, respectively, which is approximately consistent with the results of the present study. In this study, 43.12% of the birds were non-inbred and the rest of the birds (47.68%) were grouped in six different inbred groups. The results showed that among the inbred groups, the most inbred birds (47.68%) were birds with inbreeding coefficients between 0 and 5%, which indicates that the population inbreeding is relatively low. Various studies have been performed on the inbreeding rate of domestic chickens in the country; the results are consistent with the results of the present study. For example, the results of a study on 26 generations of genetically improved domestic chickens of Mazandaran showed that, the rate of inbreeding for this population is relatively low (Ghorbani and Omrani 1399). In another study to estimate the inbreeding rate of Indigenous Kokok balenggek chicken (KBC) population in Indonesia, the researchers estimated the inbreeding coefficient to be 0.0031 (0.31%) and concluded that considering the inbreeding rate of herd is less than one percent, and inbreeding less than one percent indicates that one percent of heterozygosity is lost in one generation, the population is not endangered (Rusfidra et al. 2014). In another study on the population size of 37 protected local chicken breeds in five European countries, the relatively low inbreeding rates (0.02 to 0.71%) was reported (Spalona et al. 2007).
In our study, the highest effect of individual inbreeding was shown on BW12. In this trait, for a one percent increase in inbreeding rate, the body weight decreases by 1.17 grams. Based on the another study performed on the Isfahan improved native chicken population (Jelokhani et al. 1400), it was shown that for every one percent increase in individual and maternal inbreeding, the WSM is reduced to 3.97 and 1.38 grams, respectively. In another study conducted on Mazandaran native fowls, the results showed that for every one percent increase in individual inbreeding, body weight decreased for the traits including BW12, BW8 and WSM (Ghorbani and Emrani 1399). The results of another study on the improved Fars native fowl revealed the greatest effect of inbreeding on body weight traits (8 and 12 weeks). In this study, for every one percent increase in individual inbreeding, body weight at 12 and 8 weeks decreased by 2.14 and 1.07, respectively (Jelokhani and Ghorbani, in press).

Conclusion:
According to the findings in the present study, since the population is closed and under selection, the probability of inbreeding in the population is high. However, because there are controlled matings in the population of breeding station of Khorasan Razavi indigenous fowl, the inbreeding rate in each generation has increased slightly, which shows the appropriate management practices are running in selecting the best birds. Best management practices such as controlling the matings between birds can prevent an increase in population inbreeding as well as its possible negative effects on the traits.