The climatic conditions in Croatia are deteriorating which significantly increases the frequency of heat stress. This creates a need for an adequate dairy farming strategy. The impact of heat stress can be reduced in many ways, but the best long-term solution includes the genetic evaluation and selection for heat stress resistance. In order to create the basis for genetic evaluation, this research determined the variation in daily milk yield (DMY) and somatic cell count (SCC) as well as the differences in resistance to heat stress due to production level (high, low) and breed (Holstein, Simmental) of dairy cattle breed in Croatia. For statistical analysis, 1,070,554 test-day records from 70,135 Holsteins reared on 5679 farms and 1,300,683 test-day records from 86,013 Simmentals reared on 8827 farms in Croatia provided by the Croatian Agricultural Agency were used. The results of this research indicate that the high-producing cows are much more susceptible to heat stress than low-producing especially Holsteins. Also, the results of this research indicate that Simmental breed, in terms of daily milk production and somatic cell count, could be more resistant to heat stress than Holstein. The following research should determine whether Simmentals are genetically more appropriate for the challenges that are in store for the future milk production in this region. Furthermore, could an adequate production level be achieved with Simmentals by maintaining the heat resistance?

Abstract. Aiming at the determination of the metabolic disorder prevalence as well as the effect of subclinical disorders on daily milk traits of Holsteins, over 1 million (1 962 831) test day records were analysed. The metabolic disorder (ketosis or acidosis) prevalence was indicated by the fat-to-protein (F /P ) ratio, while subclinical disorder was indicated by the F /P ratio and cows' daily production. The effect of subclinical disorders on daily milk traits was tested by Scheffe's method (SAS/STAT). The highest ketosis prevalence occurred in early lactation. In first-parity cows ketosis prevalence was highest in the first 15 days of lactation, while in cows in higher lactations, the prevalence peak occurred on the 25th day. A higher ketosis prevalence during the entire lactation was observed in multiparous cows compared to first-parity cows. Regarding the acidosis prevalence, high values were determined at the beginning of the lactation with a decreasing trend until mid-lactation, when the prevalence increased up to 22 %. The highest decrease in daily milk yield as a consequence of subclinical ketosis was determined as 4.21 and 3.72 kg day −1 in first-parity cows and those with more than four lactations, respectively. A significant negative effect of subclinical acidosis on daily milk yield (2.79 kg day −1 ) was highest in cows in the third lactation. A production decline in subsequent milk controls due to subclinical ketosis or acidosis in all cows was also determined. Subclinical disorders also significantly alter daily milk quality. This indicates that the test day records could be used as a cost-effective and non-invasive method for monitoring herd health.

Considering the rapid climate changes worldwide, with the purpose to reduce financial losses for dairy farmers and enable a more sustainable farming as well, there is an increasing necessity to implement breeding values for heat resistance in breeding strategies. The estimation of breeding values requires a determination of temperature-humidity index (THI) threshold value. Therefore, the objective of this study was to determine the temperature-humidity index (THI) threshold values for daily milk yield, daily fat and protein content of milk produced by Holsteins and dairy Simmentals in Croatian farms. With that purpose, 1.070,554 test-day records from 70.135 Holsteins reared on 5.679 farms and 1.300,683 test-day records from 86.013 Simmentals reared on 8.827 farms in Croatia were compared to records of ambient temperature and relative humidity collected in the barn, during regular milking recordings from January 2005 to December 2012. Accordingly the following conclusions could be drawn: threshold values for daily milk yield were highly dependant on the parity and breed (Holsteins: 68, 69 and 72 for 1 st , 2 nd , 3+ parity; Simmentals: 77 for 1 st ); daily fat and protein contents showed a highly significant decrease due to heat stress condition (THI in 66-80) in both, Holstein and Simmental cows, and in all parity classes; multiparous cows were more resistant to heat stress than primiparous; Simmental cows were more resistant to heat stress than Holsteins.

Farm breeding of cattle is organized so that the animals are kept in a controlled environment (an closed corner, feeding time, milking time, workers), where daily animal encounter with familiar sounds, smells, movements and equipment. Any positive or negative change of environment causes certain physiological reactions of cattle on it. Negative stimuli (relocation, new technological activities, unidentified sounds and people) disrupt the homeostasis of the animals and lead them into a state of stress. During stress in cattle appears central inhibition of milk ejection from the alveoli of the udder, causing its incomplete emptying which could lead to pathological changes. In stressful situation, there is a connection between nervous (SA system), endocrine (HPA-axis) and the immune system. Stimuli from the outside were obtained through sight, hearing, smell and touch, leading to nerve impulses which then activate the endocrine system (increased levels of cortisol, β-endorphin, adrenaline, and reducing levels of oxytocin in the blood), whose actions over a long time lead to decline in immune response and susceptibility to disease. Many studies suggest an association between central inhibition of milk ejection and stressful situations (first milking of primiparious cattle, environment changes), but the very principle of inhibition is not fully understood. Some studies indicate an association between endogenous opioids (β-endorphin) and catecholamines (adrenalin) in the central inhibition of milk ejection, however, there is still no clear evidence and there is still plenty to do in this area.