Genetic and Genomic Analysis of Milk Yield, Milk Composition and Lactation Curve Traits of Thai and Australian Dairy Cattle

The objective of this thesis is to estimate genetic parameters and discover genes and genomic regions associated with milk yield, lactation curve traits (peak milk yield, days to peak milk yield and persistency) and milk composition (fat and protein percentage) in Thai and Australian dairy cattle. The data on Thai dairy cattle from three government farms were analysed. Cumulative 305- and 100-day milk yield was calculated by the Wood model and an adapted test-interval method. The heritability of 305-day milk yield, peak milk yield and protein percentage had moderate to high estimates ranging from 0.19 to 0.45 while days to peak milk yield, persistency and fat percentage have low heritability ranging from 0.08 to 0.14. High genetic correlations were found between 305- and 100-day milk yield (0.89), 305-day milk yield and peak milk yield (0.86) and days to peak milk yield and persistency (0.99). The data on Australian dairy cattle, provided by DataGene, were used for genetic analysis of the same traits as those of the Thai dataset. In addition, genotypic data on the Australian dairy cattle were included for the analysis. The estimated pedigree-based heritability by ASReml-R and the estimated SNP-based heritability by GCTA tools were compared. The heritability estimates for most of the traits were moderate to high ranging from 0.15 to 0.56. The heritability estimates from the pedigree-based analysis were lower than the SNP-based estimates for most of the traits except persistency and fat and protein percentages. In addition, GWAS of all the traits was conducted. The most significant SNPs and genes associated with a number of lactation traits were discovered on Chromosome 14