Development of genetic and genomic breeding values for the improvement of health, functionality and milk coagulation properties in Italian Holstein cattle

In recent decades, the application of high-throughput genomic technologies has revolutionized dairy cattle breeding, enabling the simultaneous optimization of production, health, and functionality. This thesis presents a comprehensive framework for the genetic and genomic evaluation of multiple functional traits in the Italian Holstein population, aiming to provide robust decision-support tools for breeders. First, the statistical model for milkability was updated using over 7.8 million test-day records. The analysis yielded heritability and repeatability estimates of 0.28 and 0.50, respectively. Validation demonstrated a genomic reliability of 0.39, though unfavorable correlations with mastitis resistance and teat morphology highlight the need for balanced selection. Second, the genetic architecture of gestation length (GL) was characterized by partitioning direct and maternal effects. Direct heritability estimates were 0.43 for first parity and 0.35 for later parities. Correlations with milk yield and fertility suggest an intermediate optimum for GL to avoid adverse effects on productivity. Third, given that 77% of Italian milk is destined for cheese, a routine evaluation for milk coagulation properties was developed. Analyzing over 4 million records, heritabilities ranged from 0.11 to 0.33. Incorporating a mixed reference population of bulls and cows increased genomic prediction reliability by an average of 69%. Fourth, to improve udder-health assessment, a “state of infection” (SI) phenotype was introduced by combining somatic cell count (SCC) and differential SCC. Heritability for SI was estimated at 0.09, with a strong genetic correlation (0.94) to traditional SCC. Finally, a routine evaluation for twinning rate was implemented to help reduce associated health and economic burdens, despite low heritability (0.01). These studies have successfully broadened the Italian Holstein breeding program. Future expansions may include direct on-farm disease diagnoses and the implementation of single-step genomic evaluations to further enhance reliability.