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Title: Estimation of genetic parameters in dairy cattle using an animal model and implications for genetic improvement
Author: Visscher, Peter Martin
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1991
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The aim of this study was to estimate genetic parameters in the U.K. pedigree Holstein-Friesian (HF) population using an animal model (AM), and to investigate some implications of the results for genetic improvement. In a theoretical study it was shown that little bias in estimating variances components is introduced by grouping herds according to their mean (milk) production, a common practice for investigating heterogeneity of variance in dairy populations. For each of 26 large pedigree herds, comprising a total of 7720 HF cows, variances and h2 for first lactation fat yield were estimated with residual maximum likelihood (REML) using an AM. The mean fat yield was 212 kg. The mean and range of individual herd h2 estimates were 0.38 and 0.03-0.80 respectively, and the average standard error of the h2 estimates was 0.19. Using likelihood ratio (LR) tests it was found that individual herd h2 were not significantly different from each other (P> 0.05), but that phenotypic variances differed substantially among herds (P< 0.01). An investigation into the statistical power of a LR test for small samples showed that it is difficult to detect real differences in individual h2 if the standard errors of the estimates are relatively large. Using production records in lactations 1-3 from 100 large Holstein-Friesian pedigree herds, parameters for milk, fat and protein yield in lactations 1-3 were estimated with REML using an AM. The number of records for each lactation was approximately 39000, 26000 and 17000 for lactation 1, 2 and 3 respectively. Heritabilities for the three yield traits were similar: approximately 0.36 in lactation 1 and 0.30 in lactations 2 and 3. Genetic correlations between yield traits in lactations 1 and 2, for example between milk production in first and second lactations, were approximately 0.86. Genetic correlations between yield traits in lactations 2 and 3 were nearly unity. Genetic correlations between yield traits within lactations ranged from 0.58, for milk and fat yield in lactation 3, to 0.91, for milk and protein yield in lactation 1. Genetic correlations between yield traits between lactations ranged from 0.55, for milk yield in lactation 1 and fat yield in lactation 2, to 0.85, for milk yield in lactation 2 and protein yield in lactation 3. Environmental correlations between traits within lactations were approximately 0.95, and approximately 0.40 across lactations. The effect of simplifying covariance structures for milk, fat and protein yield in lactations 1-3 on accuracy of selection for lifetime yield was investigated using selection index theory. It was found that applying a transformation to make the traits in lactation 1 independent at the phenoptypic and genetic level to the yield traits in later lactations, and assuming that three new uncorrelated variates were formed, was highly efficient in terms of accuracy of selection when compared to the accuracy of a general multivariate model. This transformation was recommended for a national BLUP evaluation, since it may take account of selection to a larger extent than when performing separate analyses for milk, fat and protein yield.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available