(Co)variance component and genetic parameter estimation using a repeatability model in the Grootfontein merino stud


K.R. Nemutandani1#, M.A. Snyman1, W.J. Olivier1 & C. Visser2

1Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900, South Africa

2Department of Animal & Wildlife Sciences, Faculty of Natural & Agricultural Sciences, University of Pretoria, 0002, South Africa

# Corresponding author: Khetho Nemutandani



Background: It is important for any breeding program that the genetic and environmental components of variance for the traits under selection are known, so that accurate genetic parameters can be estimated for incorporation in genetic evaluation, selection schemes and especially genomic selection. This paper forms part of a bigger study that focuses on identifying the most appropriate models of analysis for body weights recorded at different ages in Merino sheep, which will eventually lead to the implementation of genomic selection in the South African Merino sheep breed.


Aim: To estimate (co)variance components and genetic parameters for body weight in Merino sheep using a repeatability model.


Methodologies: Data for body weight recorded at different ages in the Grootfontein Merino stud from 1968 to 2012 were included in the analysis. The traits included were birth weight, weight at 42-days of age, weaning weight, body weight at 6 months, 8 months, 12 months  and 15 months  of age and adult weights recorded on the ewe flock from 2 to 7 years of age. The PROC GLM-procedure of the SAS statistical package was used to determine the fixed effects for inclusion in the models. The estimation of the genetic parameters was done with the ASREML program by fitting a repeatability model. By including an animal permanent environmental effect, direct additive and maternal additive genetic effects, with or without a covariance between them, either including or excluding a maternal permanent environmental effect, and either including or excluding splines, 12 possible repeatability models were tested.


Results: Direct heritabilities ranged from 0.26 to 0.41 among the different models, while maternal heritability ranged from 0.01 to 0.27. Maternal and animal permanent environmental effects ranged from 0.12 to 0.15 and from 0.03 to 0.11 respectively. The direct maternal correlation ranged from
-0.89 to -0.63 among the different models fitted. Repeatabilities varied from 0.33 to 0.51 for the various models tested. The repeatability of body weight from birth weight to 68 months of age, estimated with the most suitable model, was 0.37 ± 0.04. This is low compared to the range of 0.46 to 0.75 reported in literature. 


Discussion: The model including an animal permanent environmental effect, direct additive and maternal additive genetic effects, without splines fitted, was the most suitable repeatability model for estimation of genetic parameters for body weight. The AIC and BIC concurred with LogL values that this model was the most suitable.


Conclusion: These results will be incorporated with those obtained for the analyses with REML and random regression procedures, in order to obtain the most suitable model and procedures for analyses of body weight in Merino sheep.



Proceedings 49th SASAS congress, Stellenbosch