Considering the significant influence of high ambient temperature and heat waves on the well-being and productivity of dairy cows, it is to be expected that, in the course of the next few decades, climate conditions for raising cattle will deteriorate. Research has shown that heat stress causes many negative consequences in terms of physiological and behavioural disturbances and significant losses in milk production. The effort to reduce the risk of the occurrence of heat stress among dairy cows also involves the search for new environmental methods of predicting heat stress. The aim of this paper is to review and systematise the current state of knowledge on the topic of the most widely used environmental methods of determining and predicting heat stress in dairy cows and also to show the directions of studies for the future. Based on an analysis of the most popular indexes, the study evaluated their suitability for forecasting heat stress related to maintenance systems and climate conditions for cows. However, the negative results of heat stress often appear with a delay, and a carry-over effect may be experienced (summer heat stress may affect the cows until autumn). The time of the year and breed of cows could have a big impact on when animals become sensitive to increasing heat loads. This likely can be a big contributor to the discrepancies within the different heat stress equations. It is essential to prevent the occurrence of heat stress, predicting it by observing local microclimate conditions and using meteorological forecasts. Thanks to these measures, a breeder may prepare and implement suitable solutions for protecting the animals.

ABSTRACT:The impact of low temperatures combined with higher velocity of ventilated air in winter during severe frost in a free stall barn on the development of cold stress in Holstein-Friesian breed dairy cattle was studied. The study included measurements of outside and inside air parameters and cows' milk yield. Cold stress was evaluated in three technological groups (TG) using the WCT (Wind Chill Temperature) index. During the research, significant temperature and wind velocity differences (by up to 5°C and 0.5 m/s) were recorded in three zones of the barn occupied by the individual TG. All this resulted in different values of WCT. During most severe frosts lasting for 9 days, the average air temperature in the barn was -8.9°C (the lowest value was -17.3°C). The calculated average operative temperature for cows from all TG was as follows: -7.0°C for TG1, -11.1°C for TG2, and -12.3°C for TG3. It was also observed that animals from TG2 suffered from mild cold stress, which resulted in milk yield reduction by approximately 2 kg. It was concluded that there is a strong correlation (r = 0.72-0.89 with P < 0.05) between milk production and the WCT index. The measurements of most important microclimate parameters in the barn were conducted during winter seasons over the period of 2 years. They led to the conclusion that cattle kept in free stall barns are not vulnerable to the combination of low temperature and increased air movements. Research results of the present as well as of other authors were the basis for developing a table determining operative temperature for cows depending on the temperature of the environment and air movement velocity. Yet, the calculations and analysis of results show that there is a need to improve the applied calculation formula for operative temperature during cold weather.

The presented study determining the relationship between the Temperature Humidity Index (THI) value during the summer months (June-August) and daily behaviour of 40 Holstein-Friesian dairy cows, including the length of standing, laying bouts and locomotor activity. Behaviour observations were conducted on a preferences test concerning their choice to time and place of rest with respect to time duration of THI: neutral (maximum 3 h with THI ¼ 68), warm (time of THI > 68 occurrence lasted less than 12 h) and hot (time of THI > 68 occurrence lasted longer than 12 h). Obtained results was processed using analysis of the Spearman's correlation coefficient with significant at p < .05. The results showed a decrease in daily total lying time between the neutral and hot periods from 9.9 to 7.8 h/d. Along with the increase in THI by 1 unit between the warm and hot periods, there was observed a decrease in total lying time by 38 min/d in the cubicles. With an increase in the THI was also observed an increase by 15 min/d in the lying cows in the manure alleys during the night-time. During the warm and hot periods, the cows' activity showed a negative correlation with the increasing THI. Cows changed their behaviour during the summer depending on the prevailing environmental conditions. The behavioural studies on cows, without taking into account the variability of microclimate conditions, can lead to their inaccuracy and the misrepresentation of their results. ARTICLE HISTORY

A trend of global warming has been observed over the last few years and it has often been discussed whether there is an effect on livestock. Numerous studies have been published about heat stress in cattle and its influence on the physiology and productivity of animals. Preventing the negative effects of heat stress on cattle is essential to ensure animal welfare, health and productivity. Monitoring and analysis of physiological parameters lead to a better understanding of the adaptation processes. This can help to determine the risk of climate change and its effects on performance characteristics, e.g. milk yield and reproduction. This, in turn, makes it possible to develop effective measures to mitigate the impact of heat load on animals. The aim of this article is to provide an overview of the current literature. Studies especially about the physiological and productive changes due to heat stress in cattle have been summarised in this review. The direction of future research into the aspect of heat stress in cattle is also indicated.

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