PSVIII-15 The effect of heat stress on the eating, ruminating and activity behaviour of dairy cows in early, mid, and late lactation.

Abstract This study investigates the interactive effects of heat stress and lactation stage on the behaviour of lactating Holstein Friesian cows. Understanding how environmental stressors affect different stages of lactation is crucial for improving herd management and welfare. Data were collected from 60 cows between 20 June and July 2024, resulting in a total of 1680 records measuring eating, ruminating, and inactivity times. Behavioural data were recorded using automated sensors. The cows were categorised into three groups based on lactation stage: early lactation (DIM ≤ 60), mid-lactation (DIM 61–180), and late lactation (DIM > 180). Behaviour was assessed under two thermal conditions: moderate heat stress (THI 72–76) and severe heat stress (THI ≥ 77). A linear mixed model was used for statistical analysis. The results demonstrated a significant interaction between heat stress and lactation stage on cow behaviour (p < 0.05). Eating time under moderate heat stress did not differ significantly between lactation groups. However, under severe heat stress, late-lactation cows exhibited the greatest reduction in eating time, decreasing by 60 min/day compared to reductions of 46 min/day and 54 min/day in early- and mid-lactation cows, respectively. This indicates that late-lactation cows have a lower capacity to maintain feeding behaviour under high temperatures. Ruminating time also showed significant differences across lactation stages and heat stress levels (p < 0.05). Late-lactation cows experienced the most pronounced reductions in ruminating time under both moderate and severe heat stress, with declines of 37 min/day and 18 min/day, respectively, whereas mid-lactation cows experienced smaller reductions, and early-lactation cows remained largely unaffected. This suggests that as lactation progresses, cows become more vulnerable to heat stress, which may impact rumen function and digestion efficiency. Inactivity time increased significantly (p < 0.05) as lactation progressed, particularly under severe heat stress. Late-lactation cows exhibited the highest increase in inactivity, spending an additional 95 min/day lying down compared to 74 min/day and 61 min/day in mid- and early-lactation cows, respectively. This increased inactivity could indicate a heat stress coping mechanism, where cows conserve energy and reduce heat production. These findings highlight the importance of considering lactation stage when assessing heat stress responses in dairy cows. Under severe heat stress, late-lactation cows exhibit the greatest reductions in eating and ruminating times while increasing inactivity, making them the most susceptible group. In conclusion, targeted heat stress mitigation strategies, such as improved cooling systems and dietary adjustments, should prioritise late-lactation cows to maintain optimal production and welfare in high-temperature environments.