A Genome Wide Association Study of Milk Fat Globule Size and Casein Micelle Size in Canadian Holsteins

Milk fat globule (MFG) size and casein micelle size of bovine milk influences the composition and nutritional quality of the milk as well as its technological properties. A total of 1,170 milk samples from 228 Canadian Holstein cows from 27 herds were analyzed for mean MFG size and casein micelle size. Average MFG diameter was calculated as both a volume moment mean and surface moment mean. A heritability of 0.52 was found for both volume and surface moment means. Additionally, a very high genetic correlation of 0.96 was found between the two expressions of the trait. A heritability of 0.26 was found for casein micelle size. Positive genetic correlations between casein micelle size and MFG volume and surface moment means of 0.25 and 0.27, respectively, were found. A GWAS was carried out on 50k and HD genotypes of 123 genotyped sires for casein size and 139 for MFG size using Single SNP Regression (SSR) and Generalized Quasi-Likelihood Scoring (GQLS) methods. SNPs with MAF < 5% and having excess of heterozygosity > 0.15 were excluded along with sex chromosomes resulting in 37,723 and 254,418 usable SNPs for 50k and HD panels, respectively. Regions with significant peaks were identified on multiple chromosomes for all traits. A larger number of individuals and records are required in future work to further investigate regions of interest.