The effects of microclimatic conditions and heat stress on production traits and udder health in Holstein cows in Eastern Croatia

Abstract

The aim of this study was to evaluate the effects of microclimatic factors (ambient temperature, relative humidity, and the temperature–humidity index, THI) on production traits (daily milk yield, fat and protein content) and udder health indicators (somatic cell count and mastitis prevalence) in Holstein cows in Eastern Croatia, as well as to assess the changes across heat stress categories and mastitis distribution in relation to THI classes. The study was based on 1,207,373 test-day records obtained from the Croatian national milk recording system. Microclimatic variables were classified into temperature (<5, 5–14, 15–25, >25°C), humidity (<60, 60–74, 75–85, >85%), and THI (<68, 68–71, 72–80, >80) classes. Mastitis status was defined according to the somatic cell count (healthy, subclinical, and clinical mastitis). Data were analysed using descriptive statistics, a general linear model for the fixed effects of microclimate on production and health traits, and frequency analysis for mastitis distribution within the environmental classes. The results showed that all the microclimatic factors significantly affected milk production and udder health (P<0.001). Increased temperature and THI were associated with reduced milk yield and protein content, altered fat content, and increased somatic cell count. Higher heat stress levels were also linked to a greater proportion of subclinical and clinical mastitis. Relative humidity showed a significant but less consistent effect on production and health traits. In conclusion, microclimatic conditions, particularly heat stress expressed through THI, have a strong impact on dairy cow productivity and udder health. These findings highlight the importance of environmental monitoring and management strategies to mitigate heat stress and improve dairy herd performance.