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<p>Heat stress is a major environmental constraint on dairy cattle productivity, particularly during summer months, and its impact may vary across physiological states. This study investigated temperature–humidity index (THI) associated test-day milk yield responses in Brown Swiss cattle in Türkiye. The data were obtained from the national dairy recording system and comprised 222,638 test-day observations from 10,938 Brown Swiss cows across eight herds. The THI was calculated using daily ambient temperature and relative humidity data from official meteorological records covering the period 2000–2023, with meteorological stations located within 10&#xa0;km of each farm. Linear mixed-effects models were applied, including a summer THI-class model (Model 1), a full-year THI-class model (Model 2), and a continuous THI threshold model using a predefined breakpoint at THI = 68 (Model 3). THI class was significantly associated with test-day milk yield (<i>P</i> &lt; 0.001). In the summer class-based model, milk yield generally declined with increasing THI class, with significant THI class interactions involving lactation stage and parity (<i>P</i> &lt; 0.001). Milk yield reductions under severe THI conditions increased with parity, with the largest reductions observed in higher-parity cows across lactation stages. Consistent with these findings, the continuous THI model revealed a progressive decline in milk yield of approximately 0.05–0.06&#xa0;kg per unit increase in THI (<i>P</i> &lt; 0.001). However, no additional slope-change effect was detected beyond THI = 68 (<i>P</i> = 0.681), suggesting that the relationship was more consistent with a gradual THI-associated decline than with a distinct post-threshold response within the evaluated model framework. In conclusion, elevated THI conditions were associated with reduced test-day milk yield in Brown Swiss cattle in Türkiye, with parity-related differences contributing more strongly to variation in THI-associated milk yield responses than lactation stage alone. These findings highlight the importance of seasonally targeted, stage- and parity-specific management strategies and support the use of continuous THI-based indicators in future studies evaluating thermal load responses in dairy cattle.</p>

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Effects of temperature–humidity index (THI) on test-day milk yield across lactation stages and parities in Brown Swiss cattle

  • 期刊论文

  • 发表日期：

    2026年7月20日

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  • Mesut Yıldırır

摘要

Heat stress is a major environmental constraint on dairy cattle productivity, particularly during summer months, and its impact may vary across physiological states. This study investigated temperature–humidity index (THI) associated test-day milk yield responses in Brown Swiss cattle in Türkiye. The data were obtained from the national dairy recording system and comprised 222,638 test-day observations from 10,938 Brown Swiss cows across eight herds. The THI was calculated using daily ambient temperature and relative humidity data from official meteorological records covering the period 2000–2023, with meteorological stations located within 10 km of each farm. Linear mixed-effects models were applied, including a summer THI-class model (Model 1), a full-year THI-class model (Model 2), and a continuous THI threshold model using a predefined breakpoint at THI = 68 (Model 3). THI class was significantly associated with test-day milk yield (P < 0.001). In the summer class-based model, milk yield generally declined with increasing THI class, with significant THI class interactions involving lactation stage and parity (P < 0.001). Milk yield reductions under severe THI conditions increased with parity, with the largest reductions observed in higher-parity cows across lactation stages. Consistent with these findings, the continuous THI model revealed a progressive decline in milk yield of approximately 0.05–0.06 kg per unit increase in THI (P < 0.001). However, no additional slope-change effect was detected beyond THI = 68 (P = 0.681), suggesting that the relationship was more consistent with a gradual THI-associated decline than with a distinct post-threshold response within the evaluated model framework. In conclusion, elevated THI conditions were associated with reduced test-day milk yield in Brown Swiss cattle in Türkiye, with parity-related differences contributing more strongly to variation in THI-associated milk yield responses than lactation stage alone. These findings highlight the importance of seasonally targeted, stage- and parity-specific management strategies and support the use of continuous THI-based indicators in future studies evaluating thermal load responses in dairy cattle.