Geni correlati alla persistenza della curva di lattazione nelle bovine di razza Holstein Italiana

Abstract

The persistence of the lactation curve is a very important parameter in dairy farming because of its effect on fertility, health, profitability of a farm. Furthermore, paying attention to it affects might change the management of the farm as well. Unfortunately, nowadays little is known about persistence, mainly due to a standard breeding system that relies on 305-day lactations and does not allow animals to express their persistence characteristics to the fullest. The purpose of this thesis thus, was to analyze the current level of persistence in two different herds of Holstein cattle managed under different conditions. Test day records were collected for all the lactating animals present in the two farms in a two-year period 11/19 through 11/21, which allowed to model the lactation curve. In these two farms all animals were genotyped with several different genotype panels. The genomes were thus imputed to get 80K SNPs per each animal and therefore perform a Genome Wide Association Study (GWAS). A total of about 12,993 test day records belonging to 1035 cows were collected. The ANOVAs studies were done to compare the effect of farm, parity and season and the interaction between the above-mentioned effect. These studies were necessary to correct for the environmental effect the parameters obtained from the lactation curve: persistence, peak time and peak yield. As a result, from ANOVAs, it was noted that the effect of season, farm and parity is significant for lactation persistence, the effect of parity and farm is significant for peak time and the effect of farm, parity and the interaction between season x farm is significant for peak yield. Finally, GWAS analysis was performed to observe which genes were significantly associated with those parameters. Several genomic regions and candidate genes were identified for lactation persistence, which are widely distributed across autosomal chromosomes, especially on BTA3, BTA9, BTA14, and BTA15. For peak time, four genomic regions and candidate genes were identified, distributed across autosomal chromosomes particularly on BTA6, BTA7, BTA17 and BTA18. For what concern peak yield, four regions and candidate genes were found, distributed on autosomal chromosomes specifically on BTA1, BTA4, BTA13 and BTA15. Numerous candidate genes were found: a total of 111 genes were found for potentially coding for the three analyzed traits highlighting the complex and polygenic nature of them. Lastly, a genome ontology analysis was performed to detect which pathways are commonly shared among the significant genes. Overall, pathways related to Beta1, Beta2 and Beta3 adrenergic receptor signaling pathway seems to play an important role. Interesting to notice is also that the 5HT4 type receptor mediated signaling pathway which mediate serotonin release seems to have a connection with the studied traits. In conclusion, the findings might help to further characterize the molecular mechanisms behind the phenotypic expression of lactation persistence and milk production traits such as peak time and peak yield. This result can be useful to improve the genomic evaluation of those economically relevant traits in the Holstein cattle, starting to introduce these traits into genomic indices for the selection of the best individuals.