Estimation of Inbreeding Coefficient and Its Effects on Lamb Survival in Sheep

Introduction The mating of related individuals produces an inbred offspring and leads to an increased homozygosity in the progeny, genetic variance decrease within families and increase between families. The ration of homozygosity for individuals was calculated by inbreeding coefficient. Inbred individuals may carry two alleles at a locus that are replicated from one gene in the previous generations, called identical by descent. The inbreeding coefficient should be monitored in a breeding program, since it plays an important role at decreasing of homeostasis, performance, reproduction and viability. The trend of inbreeding is an indicator for determining of inbreeding level in the herd. Inbreeding affects both phenotypic means of traits and genetic variances within population, thus it is an important factor for delimitations of genetic progress in a population. Reports showed an inbreeding increase led to decrease of phenotypic value in some of the productive and reproductive traits.
Materials and Methods In the current study, the pedigree data of 14030 and 6215 records of Baluchi and Iranblack lambs that collected from 1984 to 2011 at the Abbasabad Sheep Breeding Station in Mashhad, Iran, 3588 records of Makoei lambs that collected from 1994 to 2011 at the Makoei sheep breeding station and 6140, records of Zandi lambs that collected from 1991 to 2011 at the Khejir Sheep Breeding Station in Tehran, Iran were used to estimating the inbreeding coefficient and its effects on lamb survival in these breeds. Lamb survival trait was scored as 1 and 0 for lamb surviving and not surviving at weaning weight, respectively. Inbreeding coefficient was estimated by relationship matrix algorithm (A=TDT') methodology using the CFC software program. Effects of inbreeding coefficient on lamb survival were estimated by restricted maximum likelihood (REML) method under 12 different animal models using ASReml 3.0 computer programme. Coefficient of inbreeding for each lamb added to models as a covariate. The most appropriate model for this trait was determined by Akaike’s Information Criterion (AIC) test.
Results and Discussion The number of survival records for Baluchi, Iranblack, Makoei and Zandi sheep breeds were 10793, 4826, 3588 and 6140, respectively. The inbred individuals were 17.63, 58.25, 4.88 and 36.32 per cent for Baluchi, Iranblack, Makoei and Zandi sheep, respectively, (2473, 3620, 175 and 2230 head respectively). The mean of inbreeding coefficient for whole and inbred populations for Baluchi lambs were 0.66 and 3.73 per cent, respectively, for Iranblack lambs were 4.59 and 7.90 per cent, respectively, for Makoei lambs were 0.25 and 4.86 per cent, respectively and for Zandi lambs were 1.22 and 3.61 per cent, respectively. Maximum of inbreeding coefficient for Baluchi, Iranblack, Makoei and Zandi lambs was 31.25, 34.70, 25.00 and 31.35 per cent, respectively. The mean of lamb survival in Whole and inbred population for Baluchi lambs were 89.11 and 88.30 per cent, respectively, for Iranblack lambs were 84.44 and 83.84 per cent, respectively, for Makoei lambs were 90.40 and 86.95 per cent, respectively and for Zandi lambs were 87.37 and 86.90 per cent, respectively. The average of inbreeding coefficient for 4 breeds was increased. The estimation of positive inbreeding coefficient trend for Baluchi, Iranblack, Makoei and Zandi were 0.035±0.012, 0.31±0.03, 0.010±0.012 and 0.020±0.012 per cent on each year, respectively. The most suitable model for survival in Baluchi, Iranblack, Makoei and Zandi breeds was 7, 12, 2 and 1, respectively. The regression coefficient of inbreeding on lamb survival were -0.26±0.11, -0.35±0.11, -0.25±1.83 and -0.04±0.20 per cent for Baluchi, Iranblack, Makoei and Zandi sheep, respectively.
Conclusion The levels of inbreeding below 5% in whole population, or annual rates of inbreeding under 1% unlikely result in substantial reduction of performance and economic income in sheep production and serious genetic variation in the population. Inbreeding depression was observed for survival trait although the levels of inbreeding coefficient were acceptable in all of the breeds investigated in this study. Therefore, the general policy in the flocks should be continued to avoid mating between close relative parents and use of enough sires and dams selected per annum. Estimated inbreeding coefficients for Baluchi and Iranblack breeds showed high degree of close mating in these herd and due to the significant effect of inbreeding on survival, it is suggested that this breeding stations should use a breeding plan to avoid mating of close relative animals.