Real-time milk traits and wearable sensor-derived rumination and feeding behaviors for assessing heat stress effects in dairy cattle.

Heat stress (HS) represents a major challenge in dairy cattle production because it is detrimental to the cows' welfare, feed intake, and milk yield and composition. Occurring under high temperature and humidity and commonly quantified by the temperature-humidity index (THI), it disrupts thermoregulation through behavioral and metabolic changes, thereby compromising productivity. In this study, we used accelerometers and in-line near-infrared (NIR) sensors to monitor the effects of THI levels on feeding (eating time and rumination) and panting behaviors assessed in terms of the time expended on them, DMI, milk yield, and milk composition (expressed in percentages of fat, protein, lactose, and casein) in Holstein cows. We analyzed records collected from 1,454 Holstein cows on 4 farms in Italy between September 2023 and January 2025. Eating time, rumination, and panting were monitored daily using AfiCollar accelerometer-based wearable sensors (785,195 hourly records), and milk yield and composition were measured in-line and in real time with a NIR-AfiLab system (258,715 records). Daily THI was calculated from National Aeronautics and Space Administration Prediction of Worldwide Energy Resources (NASA POWER) data, and its effects were evaluated using segmented generalized additive mixed models with fixed effects of farm, parity, stage of lactation, and milk productivity class, and random effects of cow and test date. Increasing THI levels had a consistently negative impact across all monitored parameters, but the magnitude of these effects varied with lactation stage, parity, and production level. Eating time and rumination time progressively declined at THI above 72, with the most pronounced reductions observed between 72 and 77, depending on the specific cow group (i.e., farm, days in milk, parity, and daily milk yield) and trait of concern. Panting time increased significantly with rising THI in a biphasic pattern. Depending on the cow group evaluated, the sharp increase began at THI values between 70 and 72, and panting reached a plateau at THI values between 83 and 84.5. Dry matter intake declined markedly once THI exceeded 71.5 to 76, indicating the onset of voluntary feed restriction to limit metabolic heat production, and then plateaued near THI 88 for DIM, parity, and milk yield class and 89 for farm. Milk yield showed substantial sensitivity to increasing THI, with the overall relative reduction from the group-specific threshold between THI 72 to 74 and THI 92 ranging from 37.1% to 56.7% across cow groups. The percentages of milk fat, protein, lactose, and casein also significantly decreased as THI exceeded values between 71 and 74. However, the early reduction in milk fat, rather than being solely attributable to HS, may partly reflect seasonal patterns, such as the "spring effect." Notably, high-producing cows frequently exhibited heightened sensitivity to heat load, whereas differences across lactation stages depended on the trait evaluated. Overall, HS had a detrimental effect on the eating time and rumination behavior, panting time, DMI, and milk yield and composition of Holstein dairy cows. Harmful effects began at around THI 70 to 74, establishing these values as critical indicators for intervention. Continuous sensor-based monitoring facilitates early detection of HS, enabling timely implementation of management strategies to ensure cow welfare and sustain productivity under thermal stress.