Evaluating production range and peak yield as indicators of milk yield and udder health in Iranian Holstein cows.

In Holstein dairy cattle, increasing milk yield through genetic selection often comes at the expense of udder health. To address this challenge, we introduce a novel and practical indicator of lactation persistency—Production Range (PR)—defined as the difference between a cow’s maximum and minimum predicted daily milk yield within a biologically meaningful period. Additionally, we evaluate peak yield, a conventional measure, to compare its effectiveness with PR in predicting future performance and udder health. Using data from 61,935 cows across 32 herds in Iran, we assessed the relationships between PR, peak yield, and two key traits: 305-day milk yield and somatic cell score (SCS)1. Notably, young cows with lower PR values at the end of their first lactation showed significantly higher milk yield and lower SCS in subsequent lactations, confirming PRs potential as a powerful early-life selection criterion. In contrast, selection for higher peak yield increased milk production but was associated with elevated SCS levels, indicating a trade-off between productivity and udder health. Although the study employs straightforward statistical methods, this was a deliberate choice to ensure the field-level usability and global applicability of these indicators in practical breeding programs.