Tina Bobić

Influence of temperature-humidity index (THI) on daily production of dairy cows in Mediterranean region in Croatia

With the aim of analysis the influence of temperature-humidity index (THI) on daily performance of dairy cows in Croatian Mediterranean region, 217,509 test-day records collected from January 2005 until April 2010 were extracted from HPA (Croatian Agricultural Agency) database. Milk recording in Croatia occurs according to the alternative milk recording method every four weeks, therefore, daily production of milk yields and components was projected using projection parameters estimated in authors’ earlier research (Gantner, 2008). Logical control of data was performed according to ICAR standards (2003). The yields recorded after the 500th lactation day was deleted from the dataset. According to the parity, cows were divided into three classes (heifers, cows in second lactation and cows in third and higher lactations) while, regarding the calving date, cows were divided in two calving season subgroups. According to the test date, four measuring season subgroups were created. Based on temperature and relative humidity recorded in stable at each milking control the daily temperature-humidity index (THI) values were calculated using the equation by Kibler (1964). For estimation of the influence of exceeded THI (above critical value -72) on daily production of dairy cows statistical model that take into account effects of lactation stage, breed, calving season, measuring season as well as effect of THI group (T1 - THI under critical value; T2 - THI above critical value). The significance of differences between the means of the daily milk, fat, protein and lactose yield, as well as between the means of the daily fat, protein, and lactose content within the THI classes in regard to parity classes was tested with Scheffe test. For the statistical analysis and the figures drawing the SAS/STAT package was used (SAS Institute Inc., 2000). The exceeded THI significantly decrease daily milk, fat, protein and lactose yield in heifers and cows in third and higher lactations. In second lactation determined decrease was not significant. Highly significant decrease of daily fat and protein content was determined in all cows, regardless the parity. The daily lactose content was not significantly influenced by enhanced value of THI. Management strategies are needed to minimize heat stress and to attain optimal cows’ production in climatic condition of Croatian Mediterranean region.

Differences in response to heat stress due to production level and breed of dairy cows

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?