Investigation of heat stress tolerance of Holstein Friesian cows

Abstract

This research investigated heat stress tolerance and physiological resilience in Holstein Friesian cows under varying environmental and physiological conditions. The overall aim was to improve understanding of how heat stress affects production, reproduction, metabolism, and behaviour, and to identify sensitive biological windows as well as potential sources of resilience that could support improved management and breeding strategies in dairy cattle. Across five complementary studies, data were collected from both controlled farm conditions and large-scale field datasets, allowing evaluation of responses under realistic production systems as well as broader population-level conditions. Environmental heat load was assessed using the temperature-humidity index (THI), and animal responses were evaluated through a combination of milk production traits, metabolic hormone profiles, reproductive performance indicators, and behavioural measurements. Results from the first study showed that short-term heat stress significantly reduced milk yield and altered milk composition, particularly affecting protein and lactose content, while also increasing circulating concentrations of insulin, prolactin, and non-esterified fatty acids (NEFA). However, coat colour had no measurable effect on susceptibility to heat stress, indicating limited physiological relevance under the tested conditions. The second study demonstrated that seasonal temperature variation significantly influenced metabolic hormone concentrations, particularly insulin and prolactin, with spring and summer representing the most physiologically demanding periods for dairy cows, suggesting that spring may also be considered a heat-stress season. In contrast, NEFA levels remained relatively stable across seasons, indicating a more regulated energy mobilisation response. In contrast, NEFA levels remained relatively stable across seasons, suggesting a more regulated energy mobilisation response. In reproductive analyses, heat stress during the three weeks prior to insemination emerged as the most sensitive window, with a marked reduction in conception success. Notably, the heritability of insemination success increased under severe heat stress conditions, suggesting the presence of genetic variability in resilience to thermal stress. Behavioural responses were also strongly affected by heat stress. Severe conditions reduced eating and rumination time while increasing inactivity, with the most pronounced effects observed in late-lactation cows, indicating a stage-dependent vulnerability. Finally, prenatal heat stress during early gestation negatively affected later milk production and delayed reproductive development, highlighting early gestation as a critical developmental programming window, whereas postnatal exposure showed limited long-term effects. Overall, the findings demonstrate that heat stress exerts multi-system effects on dairy cows, influencing metabolic, reproductive, and behavioural functions in a stage-dependent manner. Importantly, the results also indicate the presence of genetic variability in heat stress resilience, providing opportunities for targeted selection. These insights offer a strong scientific basis for improving herd management, welfare strategies, and genetic selection programmes aimed at enhancing dairy cow performance under increasingly challenging thermal environments.