Impacts of Climatic Factors on Milking Frequency and Milk Yield in Automatic Milking Systems Under Hot-Humid Conditions

Heat stress in hot and humid environments can severely limit the expected advantages of automatic milking systems (AMSs). This study was conducted to investigate the environmental thresholds for milking frequency (MF) and milk yield (MY) in AMSs under hot and humid conditions. We collected milking data over a 2-year period (from 29 March 2021 to 28 March 2023) from an AMS barn with an average herd size of 20.0 ± 2.2 cows (primarily Holstein cows) to investigate the effects of the temperature–humidity index (THI), dry-bulb temperature, and relative humidity on MF and MY. Both MF and MY were significantly negatively correlated with THI, temperature, and relative humidity, while a strong positive association was observed between MF and MY. Under the herd management conditions of this farm, the THI thresholds for MY and MF were 73 and 70, respectively. Beyond these thresholds, each unit increase in the THI resulted in reductions of 0.522 kg/day in MY and 0.035 times/day in MF. The Spearman correlation coefficients revealed that the negative association between MY and THI reflected a lagged, cumulative effect of weather on MY. A management-driven change in supplementary feed from soybean hulls to concentrate feed was associated with higher adjusted MF and MY. The CF period also showed higher observed THI thresholds for MF and MY than the SH period. Overall, heat stress remained a major constraint on AMS performance under hot-humid conditions, supporting the need for effective cooling and management strategies.