Milk Components and Fatty Acid Composition as Predictors of Days Open in Early Lactation Holstein Cows.

This study investigated relationships between milk components, fatty acid (FA) composition, and days open in early lactation Holstein cows under heat stress and non-heat stress conditions, and evaluated predictive ability for reproductive performance. Data from 461 multiparous cows (1155 observations) across three farms were analyzed using dairy herd improvement (DHI) records from March 2020 to September 2024. Cows were classified by days open (≤ 150 vs. ≥151 days) and season, and analyzed with linear mixed models. Cows with shorter days open consistently exhibited higher milk protein, solids-not-fat (SNF), and de novo FA (C4:0-C14:0) within 60 days in milk (DIM), while those with longer days open showed higher fat-to-protein ratio and preformed FA (mainly ≥ C18). These trends were more pronounced under heat stress. Receiver operating characteristic analysis was performed, and the area under the curve (AUC) values showed that SNF (AUC = 0.699), milk protein (AUC = 0.676), and de novo FA (AUC = 0.670) at 31 to 60 DIM under heat stress demonstrated moderate but limited discriminative ability. Farm-level analysis indicated substantial inter-farm and within-farm variation, particularly under heat stress. Milk protein, SNF, and de novo FA may thus be potential biomarkers for prolonged days open detectable via routine DHI monitoring.