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

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.