Milking-Order Stability and Stabilization After Herd Entry Under Heat Stress: An Observational Study in a Large Dairy Herd.

Heat stress is known to affect dairy cow behavior, but its effects on milking-order patterns and stabilization after herd entry in large commercial herds are not well described. We analyzed 70,491 parlor records from 484 Holstein cows milked three times daily in a free-stall herd to examine how thermal conditions shape milking-order dynamics. Daily intraday milking-order displacement was quantified based on changes in queue position between successive milkings. Cow-specific regressions of milking order on ambient temperature were used to identify clusters of thermal response, and time to reach a stable queue position was evaluated for newly introduced cows according to thermal conditions at herd entry. Results show a modest increase in intraday physical displacement from thermally comfortable to severely hot days, whereas relative displacement did not differ significantly across the four thermal categories. Cows showed consistent individual differences in how their milking position shifted with temperature that were only weakly associated with parity, days in milk, milk yield, or panting score. In an exploratory severe versus non-severe comparison, cows entering the herd under severe thermal conditions had a lower adjusted hazard of reaching the predefined stabilization criterion (HR = 0.62, 95% CI: 0.43-0.90). Collectively, these findings indicate that milking-order patterns extracted from routine parlor records are associated with behavioral responses to thermal conditions. Their management implications, including decisions related to herd entry and cooling, require further experimental evaluation.