Effects of Thermal Variability on Milk Production Traits in Dairy Cattle Under Temperate Continental Conditions in Serbia

Milk production in dairy cattle is increasingly challenged by thermal variability. This underscores the need for reliable assessment of microclimatic conditions and their interaction with animal- and management-related factors to ensure sustainable dairy production. The aim of this study was to evaluate the effects of thermal variability and selected environmental and biological factors on key milk production traits in dairy cattle. The influence of fixed factors related to production conditions and microclimatic variability, including the Temperature–Humidity Index (THI) as an indicator of thermal variability, on daily milk yield (MY), milk fat content (MF), and milk protein content (MP) was assessed. This study used a dataset covering two observation periods of daily milk production traits in cows of different breeds (Simmental, Holstein-Friesian, Red Holstein, and Brown Swiss) reared in three regions of the Republic of Serbia (Mačva, Podunavlje, and Šumadija), enabling an assessment of thermal variability under diverse production and microclimatic conditions. The expression and variability of the investigated traits were determined using the PROC FREQ and PROC MEANS procedures, while the effects of individual factors were analysed using general linear and regression models, with results expressed as least squares means. All examined factors showed a highly significant effect on MY, MF, and MP (p < 0.0001). Milk production was highest within the THI range of 51–60, corresponding to thermoneutral conditions, whereas higher THI values (>60) reflect increasing thermal load and were associated with measurable reductions in milk yield, as confirmed by statistical analysis. These findings demonstrate that thermal variability and heat stress significantly influence milk production and composition and highlight the importance of integrating microclimatic indicators into sustainability-oriented dairy management and breeding strategies.