The effect of different levels of heat stress on the behaviour of cows at different stage of lactation.

Rising temperatures and humidity increasingly challenge the welfare and productivity of dairy cows worldwide. Behavioural changes are among the earliest responses to heat stress, reflecting animals' efforts to reduce heat load. This study investigated how heat stress and lactation stage interact to influence the behaviour of Holstein Friesian cows. Data were collected from 60 cows between 20 June and July 20, 2024, with 1680 records of eating, ruminating, and inactivity times. Cows were grouped by lactation stage: early (days in milk, (DIM) ≤ 60), mid (DIM 61-180), and late (DIM >180). Behaviour was assessed under moderate (temperature-humidity index, (THI) 72-76) and severe heat stress (THI ≥77). A significant interaction was observed between the level of heat stress and lactation stage. Under severe heat stress, eating time declined significantly across all lactation groups, with the largest reduction observed in late-lactation cows. Ruminating time decreased significantly only in late-lactation cows, while remaining relatively stable in early and mid-lactation. Conversely, inactivity time increased in all groups, with the duration of inactivity rising as lactation advanced. These results suggest that behavioural responses to heat stress intensify with advancing lactation. In conclusion, late-lactation cows are more susceptible to heat stress than those in earlier stages, indicating a need for targeted mitigation strategies to support cow welfare and productivity.