Identification of Genomic Regions Influencing N-Metabolism and N-Excretion in Lactating Holstein- Friesians.

Excreted nitrogen (N) of dairy cows contribute to environmental eutrophication. The main N-excretory metabolite of dairy cows is urea, which is synthesized as a result of N-metabolization in the liver and is excreted via milk and urine. Genetic variation in milk urea (MU) has been postulated but the complex physiology behind the trait as well as the tremendous diversity of processes regulating the N-metabolism impede the consistent determination of causal regions in the bovine genome. In order to map the genetic determinants affecting N-excretion, MU and eight other N-excretory metabolites in milk and urine were assessed in a genome-wide association study. Therefore phenotypes of 371 Holstein- Friesians were obtained in a trial on a dairy farm under near commercial conditions. Genotype data comprised SNP information of the Bovine 50K MD Genome chip (45,613 SNPs). Significantly associated genomic regions for MU concentration revealed (BTA 9), , and (both BTA 12) as putative candidates. For milk urea yield (MUY) a promising QTL on BTA 17 including emerged, whereas , , , and (BTA 23) are suggested to influence urinary urea concentration. Minor N-fractions in milk (MN) may be regulated by and (BTA 17), whilst and (BTA 5), (BTA 6), (BTA 7), (BTA 18), and (BTA 16) are suggested to have an impact on various non-urea-N (NUN) fractions excreted via urine. Our results highlight genomic regions and candidate genes for N-excretory metabolites and provide a deeper insight into the predisposed component to regulate the N-metabolism in dairy cows.