Standardization of lactation length in Sahiwal cattle using the Kaplan-Meier survival function

The present study applied Kaplan-Meier survival analysis and Cox proportional hazards modelling to establish a breed-specific lactation length standard for Sahiwal cattle and, to the best of our knowledge, represents the first application of these methods for this purpose in Sahiwal cattle. Lactation length in 549 Sahiwal cows maintained at LUVAS, Hisar, was evaluated using data collected over a 17-year period (2008-2025) through least-squares, Kaplan-Meier, and Cox proportional hazards models. Mean lactation length was 240.60 ± 2.53 days, while the Kaplan-Meier median was 251.00 ± 2.74 days, indicating a negatively skewed distribution with a small proportion of censored observations; therefore, the median provided a more robust estimate of the typical lactation length. Nearly 67% of animals completed lactation within 250 days, while only 12.6% sustained production beyond 305 days. Season and period of calving emerged as significant non-genetic determinants of lactation length, whereas parity exerted no statistically significant influence. Cows calving during April-June sustained the longest lactations, while October-November calvers were most vulnerable to early termination (HR = 1.60; 95% CI: 1.18-2.16; P = 0.002). Among the physiological variables examined, only days to attain peak yield significantly influenced the hazard of lactation termination (HR = 0.996; 95% CI: 0.994-0.999; P = 0.002), suggesting that a delayed attainment of peak yield enhances lactation persistence and lowers the risk of premature drying off. These findings support replacing the conventional 305-day lactation standard with a breed-specific 251-day reference for Sahiwal cattle, thereby improving the accuracy of genetic evaluation and selection while demonstrating the utility of survival analysis in identifying factors influencing lactation persistence and early lactation termination.