Early-life milk production traits predict lifetime milk yield and support replacement decisions in Holstein, Jersey and Crossbred dairy cows under subtropical conditions.

Improving lifetime milk yield through the early identification of superior replacement animals is a key objective of sustainable dairy production systems. This study investigated the potential of first-lactation performance indicators to predict lifetime milk yield in dairy cattle managed under subtropical conditions. Historical production records from 349 Holstein, Jersey and crossbred dairy cows maintained at a commercial dairy farm were analysed using linear mixed models, multiple regression and receiver operating characteristic analysis. Estimated lifetime milk yield (ELMY) was calculated from average daily milk yield and lactation length across recorded lactations. Breed, parity, and their interaction significantly influenced several milk production traits (P < 0.05). Holstein cows exhibited the highest (P < 0.05) milk production traits, while milk production increased with advancing parity. Breed-related differences in milk production were evident across parities, with more pronounced variation among breeds at later parities (P4-P5) for selected traits. First-lactation daily milk yield (FDMY) was strongly associated with ELMY (r = 0.849, P < 0.001) and emerged as the most important predictor of lifetime milk yield. Final prediction model incorporating FDMY and breed explained 74.7% of the variation in ELMY (adjusted R² = 0.731). Cows classified as high lifetime producers had significantly greater FDMY than medium- and low-producing cows (P < 0.001). Binary logistic regression showed that first-lactation daily milk yield independently predicted high estimated lifetime milk-yield status after adjustment for breed (OR = 2.14, 95% CI: 1.45-3.17; P < 0.001), with an overall classification accuracy of 90.6%. Receiver operating characteristic analysis demonstrated excellent discriminatory ability of FDMY (AUC = 0.917 ± 0.036, P < 0.001), with an optimal threshold of 16.0 kg/day providing 76.2% sensitivity and 93.7% specificity. These findings identify first-lactation milk yield as a promising early-life indicator of subsequent estimated lifetime milk yield.