Effects of heat stress on water intake in Holstein dairy cows: A systematic review and meta-analysis.

Water intake is a fundamental physiological requirement in dairy cows and is strongly influenced by environmental conditions. As global temperatures rise, understanding how heat stress (HS) alters drinking behavior is essential for sustaining productivity and animal welfare. A literature search was conducted in in the Web of Science and Scopus databases and included studies published between 1985 and 2024. The current systematic review and meta-analysis synthesized 17 eligible studies on lactating Holstein cows that reported quantitative water intake data under HS, evaluating their relationship with the temperature-humidity index (THI). Across studies, cows exposed to HS consumed, on average, 10 L/d more water than those under thermoneutral conditions, corresponding to a large pooled effect size (standardized mean difference = 1.1). Meta-regression further demonstrated that water intake increased by ∼1.5 L for each unit rise in THI, indicating a clear linear sensitivity of hydration behavior to thermal load. Risk of bias was assessed, and although heterogeneity was substantial, the direction of the HS effect was consistent and robust to sensitivity analyses. These findings provide the first integrated quantitative assessment of water intake responses to HS in lactating dairy cows and offer a robust basis for improving water management planning, early heat load detection, and climate resilience strategies within dairy production systems.