Abstract
The purpose of this study is to determine the effect of age of cows on variability in mastitis prevalence risk and its concomitant impacts on their successive daily milk yield by analyzing test-day records of dairy cows collected in the period 2005–2022. The results obtained indicate a correlation between the age and breed of cows enrolled in the study and mastitis prevalence risk alongside its concomitant impacts on their successive daily milk yield. The findings demonstrate a progressive increase in mastitis prevalence risk with the advancing age of cows, notably more pronounced in the Holstein breed than in the Simmental breed. The Simmental cows exhibited a lower increase in daily milk yield, experiencing a substantial decrease in overall yields (namely milk, fat, and protein) in contrast to the Holstein cows. The primiparous cows of both breeds displayed the highest total increase in daily milk yield. This study underscores the significance of monitoring dairy herd health as a pivotal component of successful dairy herd management. Moreover, the daily lactose content was found a valuable and cost-effective indicator for ensuring the economically and environmentally sustainable operations of dairy farms.