Abstract:
This study evaluated whether heat–humidity load, quantified by the Temperature–Humidity Index (THI), reduces testday
milk yield in a commercial herd of Romanian Simmental and Romanian Brown dairy cows. We conducted a
retrospective observational analysis linking milking-robot records to daily weather, constructing a 3-day centered mean
THI and a within-cow THI component. Linear mixed-effects models with a random intercept for cow were fitted,
adjusting for parity and calendar month. The annual dataset comprised 9,931 observations from 947 cows; a warmseason
subset (May–September) included 3,355 observations from 832 cows. In the full-year analysis, the within-cow
THI effect on 24-h milk yield was not statistically significant (β = −0.014 kg per +1 THI, p = 0.219). In contrast, during
the warm season, higher within-cow THI was associated with lower milk yield (β = −0.072 kg per +1 THI, p < 0.001),
corresponding to an average decrease of approximately 0.72 kg/day for a +10-unit THI increase after adjustment. These
findings indicate that heat–humidity stress imposes a seasonally concentrated penalty on production, while annual
average effects are muted once lactation structure and seasonality are controlled. Results support targeted mitigation
and THI-aware management during hot months and motivate future work integrating detailed lactation trajectories and
finer-scale microclimate measures.
