Consistency assessment of milk fat and protein percentages across 3 daily milkings in Holstein and Jersey dairy herds

Dairy cattle milking test plans in the United States and globally have evolved substantially since the 1960s toward cost-effective sampling methods. Test-day recording frequencies vary, adapting to the specific management needs of different herds. Typically, a cow is milked twice or more daily; however, milk fat and protein percentages are commonly assessed from single-milking samples. In this paper, we introduced intraclass correlation coefficients to determine the consistency of intraday milk fat and protein percentages across multiple milkings within the same cow. This metric extends beyond simple pairwise correlations, enabling robust comparisons across multiple milkings. Various forms of intraclass correlations are also demonstrated. Our results show that although protein percentages exhibit high consistency, fat percentages display notable variability throughout the test day. Hence, adjustment factors for milk fat percentage should differ according to individual milkings and consider the effects of the milking interval, DIM, and parity. Overall, the results demonstrate the utility of intraclass correlation as a consistency measure, providing a valuable tool for assessing the data quality of milk components for dairy breeding and management decisions.