Optimization of selection for growth in Menz Sheep while minimizing inbreeding depression in fitness traits
© Gizaw et al.; licensee BioMed Central Ltd. 2013
Received: 3 December 2012
Accepted: 23 May 2013
Published: 19 June 2013
The genetic trends in fitness (inbreeding, fertility and survival) of a closed nucleus flock of Menz sheep under selection during ten years for increased body weight were investigated to evaluate the consequences of selection for body weight on fitness. A mate selection tool was used to optimize in retrospect the actual selection and matings conducted over the project period to assess if the observed genetic gains in body weight could have been achieved with a reduced level of inbreeding. In the actual selection, the genetic trends for yearling weight, fertility of ewes and survival of lambs were 0.81 kg, –0.00026% and 0.016% per generation. The average inbreeding coefficient remained zero for the first few generations and then tended to increase over generations. The genetic gains achieved with the optimized retrospective selection and matings were highly comparable with the observed values, the correlation between the average breeding values of lambs born from the actual and optimized matings over the years being 0.99. However, the level of inbreeding with the optimized mate selections remained zero until late in the years of selection. Our results suggest that an optimal selection strategy that considers both genetic merits and coancestry of mates should be adopted to sustain the Menz sheep breeding program.
Research problem and hypothesis
Selective breeding is expected to reduce fitness of animal populations through its negative effect on genetic diversity, particularly in small closed populations. The reduction in fitness caused by intensive selection for production traits is more severe under random mating systems due to more frequent mating between close relatives . Optimum genetic gains while minimizing inbreeding can be achieved by adopting appropriate selection and mating designs. Such designs include the family mating design , where mating permissions are restricted within mating groups, and minimum coancestry mating, which involves individual mate selection that limits the contribution of individuals to future generations [3–5].
A selective breeding program for the Menz sheep breed in Ethiopia has been ongoing in a closed nucleus flock since 1998. Remarkable improvements in body weight have been recorded . The objectives of this paper were to evaluate the current Menz sheep breeding program in terms of genetic trends in fitness (fertility and survival), as related to inbreeding, and to investigate whether the observed genetic gains in body weight could have been achieved with a reduced level of inbreeding by optimizing the actual selection and matings conducted over the project years by adopting the individual mate selection approach.
Estimation of breeding values
A closed nucleus population of Menz sheep was established in 1998 in the subalpine highlands at the Debre Birhan Agricultural Research Center in Ethiopia. Selection of breeding stocks was based on estimated breeding values (EBV) for yearling weight which were estimated using the best linear unbiased prediction (BLUP) procedure fitting a univariate model. Mating was carried out according to a family mating design, in which random mating of individuals was allowed within mating groups [6, 7]. The mating groups were created by dividing the foundation flock into five groups at the beginning of the breeding program. The mating groups remained closed with female replacements coming from within the group, but rams were not permitted to mate in their groups of origin.
where Y i is a vector of observations for trait i, b i a vector of fixed effects for trait i (sex, season, year and dam age for yearling weight and lamb survival; season, year and age for ewe fertility), a i a vector of random animal effects for trait i, e i a vector of random residual effects for trait i and X i and Z i are incidence matrices relating records for trait i to fixed and random animal effects, respectively.
The actual matings carried out in the Menz sheep breeding program were revised in retrospect. For the optimized retrospective selection and mating, we used the actual candidates and parameters in each mating year, with selection on yearling weight and individual mate selection with minimum coancestry using the mate selection tool MateSel version 1.5 , based on the principles of optimal contributions of parents to future generations [4, 5]. Optimization was attempted by placing a hard constraint on genetic gain and constraining the threshold of progeny inbreeding to 1%. A maximum of five and a minimum of three sires (mating groups) were considered in each mating year. The average EBV and inbreeding coefficients of lambs born in each year of the optimized matings were compared with the actual values obtained in the breeding program.
Results and discussion
Observed genetic trends
Optimized genetic trends
Optimized genetic progress in yearling weight (EBV) and inbreeding (F) compared with values observed in a closed nucleus population of Menz sheep selected for yearling weight
Average EBV (kg)
Average F (%)
Percent progeny with F > 1%
Family mating has proved to be an efficient mating tool to achieve maximum gains in productive traits with minimal losses in fitness traits in the short term. However, a strategy involving optimal mate selection that considers both genetic merits and coancestry of mates should be adopted to sustain the Menz sheep breeding program in the long run.
The Menz sheep breeding program is financed by the Amhara Regional Agricultural Research Institute, Ethiopia. Contributions of the animal breeding staff of Debre Birhan Agricultural Research Centre are greatly acknowledged.
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